[msan] Enable and update neon_vst_float test case (#100435)

This enables the neon_vst_float test case
(#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.

Author: thurstond