[X86] combineMinMaxReduction - match any minmax reduction to a legal scalar (#195261)
Further relaxation of the VECREDUCE minmax folds - recognise any
>=128-bit vector that reduces to a legal scalar.
Adds custom ops for more VECREDUCE types (inc vXi32/vXi64 types)
Remaining issues are sub-128-bit vector reductions (unnecessary padding
with neutral elements) and handling of i64 types on 32-bit targets - I'm
working on fixes for both of these.