[ARM] Remove pre-UAL FLDM/FSTM aliases

These are pre-UAL syntax, and we don't support any other pre-UAL instructions,
with the exception of FLDMX/FSTMX, which don't have a UAL equivalent. Therefore
there's no reason to keep them or their AsmParser hacks around.

With the AsmParser hacks removed, the FLDMX and FSTMX instructions get the same
operand diagnostics as the UAL instructions.

Differential revision: https://reviews.llvm.org/D39196

llvm-svn: 318777