Prevent ICE in const-prop array oob check

[oli-obk]

fixes #55772
fixes #54541

[oli-obk]

not sure where highfive went. Github says...

r? @RalfJung

to be the most sensible action

[rust-highfive]

The job x86_64-gnu-llvm-5.0 of your PR failed on Travis (raw log). Through arcane magic we have determined that the following fragments from the build log may contain information about the problem.

Click to expand the log.

travis_time:end:00ff383b:start=1541704803753580481,finish=1541704806858088500,duration=3104508019
$ git checkout -qf FETCH_HEAD
travis_fold:end:git.checkout

Encrypted environment variables have been removed for security reasons.
See https://docs.travis-ci.com/user/pull-requests/#Pull-Requests-and-Security-Restrictions
$ export SCCACHE_BUCKET=rust-lang-ci-sccache2
$ export SCCACHE_REGION=us-west-1
Setting environment variables from .travis.yml
$ export IMAGE=x86_64-gnu-llvm-5.0
---

[00:03:44] travis_fold:start:tidy
travis_time:start:tidy
tidy check
[00:03:44] tidy error: /checkout/src/test/ui/consts/const-prop-ice2.rs: missing trailing newline
[00:03:44] tidy error: /checkout/src/test/ui/consts/const-prop-ice.rs: missing trailing newline
[00:03:45] some tidy checks failed
[00:03:45] 
[00:03:45] 
[00:03:45] command did not execute successfully: "/checkout/obj/build/x86_64-unknown-linux-gnu/stage0-tools-bin/tidy" "/checkout/src" "/checkout/obj/build/x86_64-unknown-linux-gnu/stage0/bin/cargo" "--no-vendor" "--quiet"
[00:03:45] 
[00:03:45] 
[00:03:45] failed to run: /checkout/obj/build/bootstrap/debug/bootstrap test src/tools/tidy
[00:03:45] Build completed unsuccessfully in 0:00:47
[00:03:45] Build completed unsuccessfully in 0:00:47
[00:03:45] Makefile:79: recipe for target 'tidy' failed
[00:03:45] make: *** [tidy] Error 1
The command "stamp sh -x -c "$RUN_SCRIPT"" exited with 2.
travis_time:start:1d9b63be
$ date && (curl -fs --head https://google.com | grep ^Date: | sed 's/Date: //g' || true)
Thu Nov  8 19:24:03 UTC 2018
---
travis_time:end:0832ae23:start=1541705043549877479,finish=1541705043556118771,duration=6241292
travis_fold:end:after_failure.3
travis_fold:start:after_failure.4
travis_time:start:1cdc734e
$ ln -s . checkout && for CORE in obj/cores/core.*; do EXE=$(echo $CORE | sed 's|obj/cores/core\.[0-9]*\.!checkout!\(.*\)|\1|;y|!|/|'); if [ -f "$EXE" ]; then printf travis_fold":start:crashlog\n\033[31;1m%s\033[0m\n" "$CORE"; gdb --batch -q -c "$CORE" "$EXE" -iex 'set auto-load off' -iex 'dir src/' -iex 'set sysroot .' -ex bt -ex q; echo travis_fold":"end:crashlog; fi; done || true
travis_fold:end:after_failure.4
travis_fold:start:after_failure.5
travis_time:start:0054cac8
travis_time:start:0054cac8
$ cat ./obj/build/x86_64-unknown-linux-gnu/native/asan/build/lib/asan/clang_rt.asan-dynamic-i386.vers || true
cat: ./obj/build/x86_64-unknown-linux-gnu/native/asan/build/lib/asan/clang_rt.asan-dynamic-i386.vers: No such file or directory
travis_fold:end:after_failure.5
travis_fold:start:after_failure.6
travis_time:start:1a437dac
$ dmesg | grep -i kill

I'm a bot! I can only do what humans tell me to, so if this was not helpful or you have suggestions for improvements, please ping or otherwise contact @TimNN. (Feature Requests)

[RalfJung]

Looks good! r=me with tidy satisfied.

[oli-obk]

@bors r=RalfJung

[bors]

📌 Commit 1206549 has been approved by RalfJung