[mips] Honour -mno-odd-spreg for vector insert/extract when MSA is enabled.

Summary:
-mno-odd-spreg prohibits the use of odd-numbered single-precision floating
point registers. However, vector insert/extract was still using them when
manipulating the subregisters of an MSA register. Fixed this by ensuring
that insertion/extraction is only performed on even-numbered vector
registers when -mno-odd-spreg is given.

Reviewers: vmedic, sstankovic

Reviewed By: sstankovic

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D7672

llvm-svn: 230235