[mips] [IAS] Slightly improve shift instruction generation in expandLoadImm.

Summary:
Generate one DSLL32 of 0 instead of two consecutive DSLL of 16.
In order to do this I had to change createLShiftOri's template argument from a bool to an unsigned.

This also gave me the opportunity to rewrite the mips64-expansions.s test, as it was testing the same cases multiple times and skipping over other cases.
It was also somewhat unreadable, as the CHECK lines were grouped in a huge block of text at the beginning of the file.

Reviewers: dsanders

Reviewed By: dsanders

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D8974

llvm-svn: 236311