[ARM] MVE VCVT lowering for f32->f16 truncs

This adds code to lower f32 to f16 fp_trunc's using a pair of MVE VCVT
instructions. Due to v4f16 not being legal, fp_round are often split up
fairly early. So this reconstructs the vcvt's from a buildvector of
fp_rounds from two vector inputs. Something like:

BUILDVECTOR(FP_ROUND(EXTRACT_ELT(X, 0),
            FP_ROUND(EXTRACT_ELT(Y, 0),
            FP_ROUND(EXTRACT_ELT(X, 1),
            FP_ROUND(EXTRACT_ELT(Y, 1), ...)

It adds a VCVTN node to handle this, which like VMOVN or VQMOVN lowers
into the top/bottom lanes of an MVE instruction.

Differential Revision: https://reviews.llvm.org/D81139