[FastISel][AArch64] Use the target-dependent selection code for shifts first.

This uses the target-dependent selection code for shifts first, which allows us
to create better code for shifts with immediates and sign-/zero-extend folding.

Vector type are not handled yet and the code falls back to target-independent
instruction selection for these cases.

This fixes rdar://problem/17907920.

llvm-svn: 216985