[NFC][PowerPC] Return early if the element type is not byte-sized in combineBVOfConsecutiveLoads

Summary:
Based on the Eli Friedman's comments in https://reviews.llvm.org/D60811 , we'd better return early if the element type is not byte-sized in `combineBVOfConsecutiveLoads`.

Reviewed By: efriedma
Differential Revision: https://reviews.llvm.org/D61076

llvm-svn: 359764