[PowerPC] Fix failure with STBRX when store is narrower than the bswap

Fixes a crash caused by r296811 by truncating the input of the STBRX node
when the bswap is wider than i32.

Fixes https://bugs.llvm.org/show_bug.cgi?id=32140

Differential Revision: https://reviews.llvm.org/D30615

llvm-svn: 297001