[dev.simd] simd: fix arm64 test build for TruncTo and uint64 shifts

Two recent renames left the arm64 test tree failing 'go vet simd/...':

1. The 'TruncateToX -> TruncToX' rename in CL 783001 (commit
   3e04a409d1) was applied as a sed-style replacement on identifiers
   that already started with 'TruncateTo', producing 'TruncToTo*'
   (note the doubled 'To') in the hand-authored TestTruncateArm64.
   The methods on the archsimd types are spelled TruncToInt8/16/32
   and TruncToUint8/16/32; replace the six call sites in
   arm64_convert_test.go with the correct names.

2. The 'shift and rotate take uint64' change in CL 783003 (commit
   fed139a1b3) widened the shift-amount parameter on every Int*/Uint*
   shift method to uint64, but left the arm64 test helpers and slice
   simulators on uint8. Update them to match:

   - _gen/tmplgen/main.go shiftConstTestTemplateArm64 generates
     uint64-typed test helpers (function signature and the inner
     amt-range slice).
   - arm64_shift_helpers_test.go is the corresponding regenerated
     output.
   - simulation_helpers_test.go shift{Left,Right}ByConstSlice and
     shiftLeftSaturating{,U}ByConstSlice now take uint64.
   - arm64_shift_test.go testShiftConstAmt and the
     TestConcatShiftBytesRight closures (csbr, hide, shift loop) take
     uint64 to match ConcatShiftBytesRight's widened parameter.

After this change 'go vet simd/...' is clean and 'go test simd/...'
passes on linux/arm64.

Change-Id: I106f6e1d7e8df70201f3969b193747c812984805
Reviewed-on: https://go-review.googlesource.com/c/go/+/783880
Reviewed-by: David Chase <drchase@google.com>
LUCI-TryBot-Result: golang-scoped@luci-project-accounts.iam.gserviceaccount.com <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Junyang Shao <shaojunyang@google.com>
Reviewed-by: Junyang Shao <shaojunyang@google.com>