[InstCombine] Extend SVEVectorFuseMulAddSub to support newly added "undef" intrinsics.

D143767 will change the intrinsics used to lower floating-point
svadd_x, svmul_x and svsub_x builtins. This will result in the
combines added as part of D140200 to no longer fire in all cases.
This patch extends the existing combines for contraction to cover
fadd_u, fmul_u and fsub_u intrinsics.

Differential Revision: https://reviews.llvm.org/D144413