[mips] Honour -mno-odd-spreg for vector splat (again)
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.
A previous commit was missing the test case and had another differential
in it.
Reviewers: slthakur
Differential Revision: https://reviews.llvm.org/D28373
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@291566
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