[AArch64] Select saturating Neon instructions

This adds some extra patterns to select AArch64 Neon SQADD, UQADD, SQSUB
and UQSUB from the existing target independent sadd_sat, uadd_sat,
ssub_sat and usub_sat nodes.

It does not attempt to replace the existing int_aarch64_neon_uqadd
intrinsic nodes as they are apparently used for both scalar and vector,
and need to be legal on scalar types for some of the patterns to work.
The int_aarch64_neon_uqadd on scalar would move the two integers into
floating point registers, perform a Neon uqadd and move the value back.
I don't believe this is good idea for uadd_sat to do the same as the
scalar alternative is simpler (an adds with a csinv). For signed it may
be smaller, but I'm not sure about it being better.

So this just adds some extra patterns for the existing vector
instructions, matching on the _sat nodes.

Differential Revision: https://reviews.llvm.org/D69374