[AArch64] Add S/UQXTRN tablegen patterns.

This adds simple patterns for signed and unsigned saturating extract
narrow instructions. They combine a min/max/truncate into a single
instruction, providing that the immediates on the min/max are correct
for the saturation type. This is just handled in tablegen with some
extra patterns.

v2i64->v2i32 is not handled here as the min/max nodes are not legal,
making the lowering quite different.

Differential Revision: https://reviews.llvm.org/D103263