correct suffix matching to search for s/l/t suffixes on

floating point stack instructions instead of looking for b/w/l/q.

This fixes issues where we'd accidentally match fistp to fistpl,
when it is in fact an ambiguous instruction.

This changes the behavior of llvm-mc to reject fstp, which was the
correct fix for rdar://8456389:
t.s:1:1: error: ambiguous instructions require an explicit suffix (could be 'fstps', 'fstpl', or 'fstpt')
fstp	(%rax)

it also causes us to correctly reject fistp and fist, which addresses
PR8528:

t.s:2:1: error: ambiguous instructions require an explicit suffix (could be 'fistps', or 'fistpl')
fistp (%rax)
^
t.s:3:1: error: ambiguous instructions require an explicit suffix (could be 'fists', or 'fistl')
fist (%rax)
^

Thanks to Ismail Donmez for tracking down the issue here!

llvm-svn: 118346