[msan][NFC] Rename NEON tests and fix comment

[thurstond]

This renames the neon_* tests to be more consistent with the original files that they were forked from. This makes it easier for maintainers to see which test files may need to be ported to MSan.

It also fixes the header comment for llvm/test/Instrumentation/MemorySanitizer/AArch64/arm64-vcvt.ll (instrumentation was updated in #126136 aka e9e6ba6, but the comment was not).

[github-actions]

⚠️ undef deprecator found issues in your code. ⚠️

You can test this locally with the following command:

git diff -U0 --pickaxe-regex -S '([^a-zA-Z0-9#_-]undef[^a-zA-Z0-9_-]|UndefValue::get)' 86f0e6dc1177fe45ec6b0ed0707053b28c227321 7eca561dd63f2c7ce8ab1858f12ef6c40972b5f4 llvm/test/Instrumentation/MemorySanitizer/AArch64/arm64-vcvt.ll llvm/test/Instrumentation/MemorySanitizer/AArch64/arm64-st1.ll llvm/test/Instrumentation/MemorySanitizer/AArch64/arm64-st1_lane.ll llvm/test/Instrumentation/MemorySanitizer/AArch64/arm64-st1_origins.ll llvm/test/Instrumentation/MemorySanitizer/AArch64/arm64-tbl.ll llvm/test/Instrumentation/MemorySanitizer/AArch64/arm64-vmul.ll

The following files introduce new uses of undef:

• llvm/test/Instrumentation/MemorySanitizer/AArch64/arm64-st1.ll
• llvm/test/Instrumentation/MemorySanitizer/AArch64/arm64-vmul.ll

Undef is now deprecated and should only be used in the rare cases where no replacement is possible. For example, a load of uninitialized memory yields undef. You should use poison values for placeholders instead.

In tests, avoid using undef and having tests that trigger undefined behavior. If you need an operand with some unimportant value, you can add a new argument to the function and use that instead.

For example, this is considered a bad practice:

define void @fn() {
  ...
  br i1 undef, ...
}

Please use the following instead:

define void @fn(i1 %cond) {
  ...
  br i1 %cond, ...
}

Please refer to the Undefined Behavior Manual for more information.