[ARM][NFC] codegen tests cleanup: remove dangling check prefixes

I am working on making FileCheck stricter (in D54769 and D53710) so that it
issues diagnostics when there's something wrong with tests.

This is a cleanup for dangling prefixes in the ARM codegen tests, e.g.:

--check-prefixes=A,B

where A occurs in the check file, but B doesn't. This can be innocent if A does
all the required checking, but can also be a bug in that test if it results in
the test actually not checking anything (if A for example only checks a common
label). Test CodeGen/ARM/smml.ll is such an example.

Differential Revision: https://reviews.llvm.org/D54842

llvm-svn: 347487