[TargetInstrInfo] replace redundant expression in getMemOpBaseRegImmOfs

Summary:
The expression for computing the return value of getMemOpBaseRegImmOfs has only
one possible value. The other value would result in a return earlier in the
function. This patch replaces the expression with its only possible value.

Reviewers: sanjoy

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D27437

llvm-svn: 290133