JIT: Assume allocations succeed (#82961)

We call a NOMEM function when allocations fail that is already marked as
no-return, but at least MSVC is not able to pick up on this fact. The
result is that all operator new calls end up with an unnecessary null
check. This adds a COMPILER_ASSUME to allow compilers to recognize
that allocations never fail. 

There was also a workaround for an old VSW bug that I've removed.

src/coreclr/inc/unreachable.h

@@ -9,14 +9,7 @@
 #define __UNREACHABLE_H__
 
 #if defined(_MSC_VER) || defined(_PREFIX_)
-#if defined(TARGET_AMD64)
-// Empty methods that consist of UNREACHABLE() result in a zero-sized declspec(noreturn) method
-// which causes the pdb file to make the next method declspec(noreturn) as well, thus breaking BBT
-// Remove when we get a VC compiler that fixes VSW 449170
-# define __UNREACHABLE() do { DebugBreak(); __assume(0); } while (0)
-#else
-# define __UNREACHABLE() __assume(0)
-#endif
+#define __UNREACHABLE() __assume(0)
 #else
 #define __UNREACHABLE() __builtin_unreachable()
 #endif

src/coreclr/jit/alloc.h

@@ -235,6 +235,12 @@ class CompAllocator
 
         void* p = m_arena->allocateMemory(count * sizeof(T));
 
+#ifndef DEBUG
+        // Could be in DEBUG too, but COMPILER_ASSUME in debug builds has extra
+        // cruft in it that we'd like to avoid in checked builds.
+        COMPILER_ASSUME(p != nullptr);
+#endif
+
         // Ensure that the allocator returned sizeof(size_t) aligned memory.
         assert((size_t(p) & (sizeof(size_t) - 1)) == 0);