[FastISel][AArch64] Use a new helper function to determine if a value type is supported. NFCI.

FastISel for AArch64 supports more value types than are actually legal. Use a
dedicated helper function to reflect this.

It is very similar to the isLoadStoreTypeLegal function, with the exception
that vector types are not supported yet.
[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.