[SystemZ] Bugfix in adjustSubwordCmp()

adjustSubwordCmp() should not optimize a load of an i1 value. This is
achieved by checking that the size and store-size of the MemoryVT are the
same.

Fixes https://bugs.llvm.org/show_bug.cgi?id=45511.

Review: Ulrich Weigand

Differential Revision: https://reviews.llvm.org/D78187