[LegalizeTypes] Add SoftenFloatOp_Unary to reduce some duplication for...

[LegalizeTypes] Add SoftenFloatOp_Unary to reduce some duplication for softening LRINT/LLRINT/LROUND/LLROUND

Summary: This will be enhanced in a follow up to add strict fp support

Reviewers: efriedma

Reviewed By: efriedma

Subscribers: hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D70751