[FPEnv] Remove unnecessary rounding mode argument for constrained intrinsics

The following intrinsics currently carry a rounding mode metadata argument:

    llvm.experimental.constrained.minnum
    llvm.experimental.constrained.maxnum
    llvm.experimental.constrained.ceil
    llvm.experimental.constrained.floor
    llvm.experimental.constrained.round
    llvm.experimental.constrained.trunc

This is not useful since the semantics of those intrinsics do not in any way
depend on the rounding mode. In similar cases, other constrained intrinsics
do not have the rounding mode argument. Remove it here as well.

Reviewed By: craig.topper

Differential Revision: https://reviews.llvm.org/D71218