[mips] [IAS] Inline assemble-time shifting out of createLShiftOri. NFC.

Summary:
Do the assemble-time shifts from createLShiftOri at the source, which groups all the shifting together, closer to the main logic path, and
store the results in concisely-named variables to improve code clarity.

Reviewers: dsanders

Reviewed By: dsanders

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D8973

llvm-svn: 236096