Modernize repmovsb implementation of x86 memcpy and allow runtime sizes.

Summary: This is a prerequisite to RFC http://lists.llvm.org/pipermail/llvm-dev/2019-April/131973.html

Reviewers: courbet

Subscribers: hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D61593

Fix typo.

Turn this patch into an NFC.

Addressing comments

llvm-svn: 360050