[mips] Prevent shrink-wrap for BuildPairF64, ExtractElementF64 when they use $sp

For a certain combination of options, BuildPairF64_{64}, ExtractElementF64{_64}
may be expanded into instructions using stack.
Add implicit operand $sp for such cases so that ShrinkWrapping doesn't move
prologue setup below them.

Fixes MultiSource/Benchmarks/MallocBench/cfrac for
'--target=mips-img-linux-gnu -mcpu=mips32r6 -mfpxx -mnan=2008'
and
'--target=mips-img-linux-gnu -mcpu=mips32r6 -mfp64 -mnan=2008 -mno-odd-spreg'.

Differential Revision: https://reviews.llvm.org/D50986

llvm-svn: 340927