Changes

IR: remove "else" after "return". NFCI.

[ARM,MVE] Fix valid immediate range for vsliq_n.
In common with most MVE immediate shift instructions, the left shift 
takes an immediate in the range [0,n-1], while the right shift takes one
in the range [1,n]. I had absent-mindedly made them both the latter.
While I'm here, I've added a set of regression tests checking both ends
of the immediate range for a representative sample of the immediate
shifts.

[ARM,MVE] Add missing IntrNoMem flag on IR intrinsics.
A lot of the IR-level intrinsics we've been defining for MVE recently 
accidentally had `props = []` instead of `props = [IntrNoMem]`, so that
optimization would have been overcautious about reordering them.
All the affected cases were due to instantiating the multiclasses
`MVEPredicated` and `MVEMXPredicated` without filling in the `props` 
parameter, because I //thought// I remembered having set the defaults in
those multiclasses to `[IntrNoMem]`. In fact I hadn't done that. Now I
have.
(The IR intrinsics that //do// read and write memory are all explicitly
marked as `[IntrReadMem]` or `[IntrWriteMem]` already, so they will
override these defaults.)

AMDGPU: Add register class to DS_SWIZZLE_B32 pattern
Reduces diff for a future patch.

TableGen/GlobalISel: Fix slightly wrong generated comment

AMDGPU/GlobalISel: Fix add of neg inline constant pattern

AMDGPU/GlobalISel: Add equiv xform for bitcast_fpimm_to_i32
Only partially fixes one pattern import.

AMDGPU/GlobalISel: Add selectVOP3Mods_nnan
This doesn't enable any new imports yet, but moves the fmed patterns 
from failing on this to hitting the "complex suboperand referenced more
than once" limitation in tablegen.

AMDGPU/GlobalISel: Add IMMPopCount xform
Partially fixes BFE pattern import.

AMDGPU/GlobalISel: Fix import of zext of s16 op patterns

AMDGPU: Use new PatFrag system for d16 load nodes

AMDGPU: Use new PatFrag system for d16 stores

AMDGPU: Eliminate more legacy codepred address space PatFrags
These should now be limited to R600 code.

AMDGPU/GlobalISel: Fix import of integer med3
This isn't too useful now, since nothing is currently trying to form 
min/max from cmp+select.