[mips] Honour -mno-odd-spreg for vector splat

Previous the lowering of FILL_FW would use the MSA128W register class when
performing a vector splat. Instead it should be honouring -mno-odd-spreg and
only use the even registers when performing a splat from word to vector
register.

Logical follow-on from r230235.

This fixes PR/31369.

Reviewers: slthakur

Differential Revision: https://reviews.llvm.org/D28373

llvm-svn: 291556