[msan][NFCI] Rename tests and update comment

This renames the neon_* tests to be more consistent with the original
file that they were forked from.

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
 ;