This addresses a TODO where previously scalarizeBinopOrCmp
conservatively bailed if one of the operands was a load.
getVectorInstrCost was updated to take in values in
https://reviews.llvm.org/D140498 so we can pass in the scalar value to
be inserted, which should return an accurate cost for a gather.
To prevent regressions on x86 this tries to constant fold NewVecC up
front so we can pass it into TTI and get a more accurate cost.
We want to remove this restriction on RISC-V since this is always
profitable whether or not the scalar is a load.
97f6076ded