[AArch64] Look through bitcast when looking for extract_high subvector

Since D61806, DAGCombiner has folded subvector_extract(bitcast(..)) to
bitcast(subvector_extract(..)), which would place a bitcast between a
subvector_extract and the operation that could be converted to a high
neon instruction (like smull2). This adds better matching for the
subvector_extract, through the tablegen extract_high PatFrags to
optionally skip the bitcast under little ending, still matchings an
extract of the high half of the input vector.

I didn't update the extract_high of a duplicate patterns, as the
ComplexPattern need names operands. I did add a extract_high_dup_v8i16
PatFrag to abstract away the common code, which can be extended in a
future patch.

Differential Revision: https://reviews.llvm.org/D126782