[TargetInstrInfo] Rename getLdStBaseRegImmOfs and implement for x86.

Summary:

TargetInstrInfo::getLdStBaseRegImmOfs to
TargetInstrInfo::getMemOpBaseRegImmOfs and implement for x86.  The
implementation only handles a few easy cases now and will be made more
sophisticated in the future.

This is NFCI: the only user of `getLdStBaseRegImmOfs` (now
`getmemOpBaseRegImmOfs`) is `LoadClusterMotion` and `LoadClusterMotion`
is disabled for x86.

Reviewers: reames, ab, MatzeB, atrick

Reviewed By: MatzeB, atrick

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D10199

llvm-svn: 239741