TableGen/GlobalISel: Fix handling of truncstore patterns

This was failing to import the AMDGPU truncstore patterns. The
truncating stores from 32-bit to 8/16 were then somehow being
incorrectly selected to a 4-byte store.

A separate check is emitted for the LLT size in comparison to the
specific memory VT, which looks strange to me but makes sense based on
the hierarchy of PatFrags used for the default truncstore PatFrags.

llvm-svn: 366129