cmd/compile/internal/ssa: move Op to op package

This change makes an op package with the Op type, the op definitions, and
supporting types. It exports the names of the types and fields to make that
possible.

This CL adds name stutter just like the block one did. We'll want to fix
that in a later CL. Unfortunately it's not as easy as just removing the
op prefix because if we removed the Op in op.Op386ORL for instance the name
op.386ORL would start with a digit.

For #80409

[git-generate]
cd src/cmd/compile/internal/ssa

rf "mv AuxType_ AuxTypeNone Op OpInvalid OpInfo OutputInfo RegInfo OpcodeTable SymEffect SymRead RegMask InputInfo OutputInfo Register ./ssaop"
cd ssaop
rf '
mv AuxType_ AuxType
mv regalloc.go regmask.go
rm regmask.go:/Register allocation/-0,/TODO: maybe we should/+0
'

cd ../_gen
rf 'sub splitPhase var splitPhase = phase1Op'
sed 's/= Op/= ssaop.Op/g' -i *.rules
sed 's/\[Op/\[ssaop.Op/g' -i *.rules

# Update and re-run simdgen
cd ../../../../../simd/archsimd/_gen
sed -i 's/^var splitPhase = .*$/var splitPhase = phase1Op/' \
    simdgen/main.go wasmgen/main.go tmplgen/main.go
sed 's/OpARM64LessThanU/ssaop.OpARM64LessThanU/g' -i simdgen/ops/ShiftRotate/go_arm64.yaml
go run . -tmplgen -simdgen -wasmgen

Change-Id: I25911b07dabad30603f92d105538a4ba6a6a6964
Reviewed-on: https://go-review.googlesource.com/c/go/+/803781
LUCI-TryBot-Result: golang-scoped@luci-project-accounts.iam.gserviceaccount.com <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Keith Randall <khr@google.com>
Reviewed-by: Michael Matloob <matloob@google.com>
Reviewed-by: Keith Randall <khr@golang.org>