[PowerPC] Fix inconsistent ImmMustBeMultipleOf for same instruction

Summary:
There are 4 instructions which have Inconsistent ImmMustBeMultipleOf in the
function PPCInstrInfo::instrHasImmForm, they are LFS, LFD, STFS, STFD.
These four instructions should set the ImmMustBeMultipleOf to 1 instead of 4.

Reviewed By: steven.zhang

Differential Revision: https://reviews.llvm.org/D54738

llvm-svn: 348109