[DAGCombine] visitEXTRACT_SUBVECTOR - 'little to big' extract_subvector(bitcast()) support

This moves the X86 specific transform from rL364407
into DAGCombiner to generically handle 'little to big' cases
(for example: extract_subvector(v2i64 bitcast(v16i8))). This
allows us to remove both the x86 implementation and the aarch64
bitcast(extract_subvector(bitcast())) combine.

Earlier patches that dealt with regressions initially exposed
by this patch:
rG5e5e99c041e4
rG0b38af89e2c0

Patch by: @RKSimon (Simon Pilgrim)

Differential Revision: https://reviews.llvm.org/D63815