[ExpandMemCmp] Correctly set alignment of generated loads

Summary:
This is a part of the series of efforts for correcting alignment of memory operations.
(Another related bugs: https://bugs.llvm.org/show_bug.cgi?id=44388 , https://bugs.llvm.org/show_bug.cgi?id=44543 )

This fixes https://bugs.llvm.org/show_bug.cgi?id=43880 by giving default alignment of loads to 1.

The test CodeGen/AArch64/bcmp-inline-small.ll should have been changed; it was introduced by https://reviews.llvm.org/D64805 . I talked with @evandro, and confirmed that the test is okay to be changed.
Other two tests from PowerPC needed changes as well, but fixes were straightforward.

Reviewers: courbet

Reviewed By: courbet

Subscribers: nlopes, gchatelet, wuzish, nemanjai, kristof.beyls, hiraditya, steven.zhang, danielkiss, llvm-commits, evandro

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D76113