[Intrinsic] Rename flt.rounds intrinsic to get.rounding

Address the inconsistency between FLT_ROUNDS_ and SET_ROUNDING SDAG
node. Rename FLT_ROUNDS_ to GET_ROUNDING and add llvm.get.rounding
intrinsic to replace flt.rounds.

Reviewed By: nikic

Differential Revision: https://reviews.llvm.org/D139507