MemCpyOpt cannot use ABI alignment even if it was not given

Summary: This patch fixes https://bugs.llvm.org/show_bug.cgi?id=44388 which incorrectly assigns an ABI alignment to memset when there was no explicit alignment given.

Reviewers: gchatelet, lenary, nikic

Reviewed By: nikic

Subscribers: hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D74083