[GlobalISel] Add G_VECREDUCE_* opcodes for vector reductions.

These mirror the IR and SelectionDAG intrinsics & nodes.

Opcodes added:
G_VECREDUCE_SEQ_FADD
G_VECREDUCE_SEQ_FMUL
G_VECREDUCE_FADD
G_VECREDUCE_FMUL
G_VECREDUCE_FMAX
G_VECREDUCE_FMIN
G_VECREDUCE_ADD
G_VECREDUCE_MUL
G_VECREDUCE_AND
G_VECREDUCE_OR
G_VECREDUCE_XOR
G_VECREDUCE_SMAX
G_VECREDUCE_SMIN
G_VECREDUCE_UMAX
G_VECREDUCE_UMIN

Differential Revision: https://reviews.llvm.org/D88750