[msan][NFC] Rename NEON tests and fix comment (#127926)

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).

Author: thurstond

llvm/test/Instrumentation/MemorySanitizer/AArch64/neon_vst.ll renamed to llvm/test/Instrumentation/MemorySanitizer/AArch64/arm64-st1.ll

@@ -2,12 +2,6 @@
 ; RUN: opt < %s -passes=msan -S | FileCheck %s
 ;
 ; Suboptimally handled by visitInstruction: llvm.aarch64.neon.
-; - fcvtas, fcvtau
-; - fcvtms, fcvtmu
-; - fcvtns, fcvtnu
-; - fcvtps, fcvtpu
-; - fcvtzs, fcvtzu
-; - fcvtxn
 ; - vcvtfp2fxs, vcvtfp2fxu
 ; - vcvtfxs2fp, vcvtfxu2fp
 ;