The patch mainly does two things. The first is allowing scalable vector
ISD::STRICT_FP_EXTEND. The second is making RISC-V customized lower
strict_fpextend to riscv_strict_fpextend_vl, the strict version of
riscv_fpextend_vl.
Reviewed By: craig.topper
Differential Revision: https://reviews.llvm.org/D145548