[mips][msa] Fix msa_[st/ld] offset check

This patch fixes a minimum divider for offset in intrinsics
msa_[st/ld]_[b/h/w/d], when value is known in compile time.

Differential revision: https://reviews.llvm.org/D54038


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@346302 91177308-0d34-0410-b5e6-96231b3b80d8