Extend the existing "scalarize" function which is used for the
fp-integer conversion instruction expansion to BinaryOperator
instructions and reuse it for the frem expansion; a similar function
for scalarizing BinaryOperator instructions exists in the ExpandLargeDivRem
pass and this change is a step towards merging that pass with ExpandFp.
Further refactoring: Scalarize directly instead of using the
"ReplaceVector" as a worklist, rename "Replace" vector to "Worklist",
and hoist a check for unsupported scalable vectors to the top of the
instruction visiting loop.
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