[msan] Enable and update neon_vst_float test case

This enables the neon_vst_float test case (llvm#100210). Although MSan does not yet generate useful IR, since opt is run with -disable-verify, the test case should still run successfully.

This patch also makes minor fixes to the test case:
- 'ptr %a' is renamed to 'ptr %p' because update_test_checks.py is case-insensitive, and was unexpectedly aliasing the %A and %a in the expected output ('<1 x double> [[A]], <1 x double> [[B]], ptr [[A]]').
- The sample output for st1x{2,3,4} was previously accidentally generated using a prototype version of MSan rather than trunk; these instructions are not yet instrumented.
- Changes the comment on how the test case was generated, because '; | sed -r 's/^\/\/ CHECK:[ ]*//'' was being interpreted by FileCheck to be a CHECK: command.

llvm/test/Instrumentation/MemorySanitizer/AArch64/neon_vst_float.ll

@@ -5,8 +5,6 @@
 ;
 ; RUN: opt < %s -passes=msan -S -disable-verify | FileCheck %s
 ;
-; UNSUPPORTED: {{.*}}
-;
 ; Generated with:
 ;     grep call clang/test/CodeGen/aarch64-neon-intrinsics.c \
 ;         |  grep 'neon[.]st'                                \