[dev.simd] cmd/compile: widen index for simd intrinsics jumptable

Feeding an unconverted uint8 to the jumptable can cause
problems either in constant propagation or later at runtime,
depending on details of the input code.

Change-Id: I5fa2299a77a73172349a165f773cf9d1198212bc
Reviewed-on: https://go-review.googlesource.com/c/go/+/702755
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Junyang Shao <shaojunyang@google.com>

src/cmd/compile/internal/ssagen/intrinsics.go

@@ -1697,11 +1697,13 @@ func immJumpTable(s *state, idx *ssa.Value, intrinsicCall *ir.CallExpr, genOp fu
 	// Make blocks we'll need.
 	bEnd := s.f.NewBlock(ssa.BlockPlain)
 
-	t := types.Types[types.TUINT8]
 	if !idx.Type.IsKind(types.TUINT8) {
 		panic("immJumpTable expects uint8 value")
 	}
+
 	// We will exhaust 0-255, so no need to check the bounds.
+	t := types.Types[types.TUINTPTR]
+	idx = s.conv(nil, idx, idx.Type, t)
 
 	b := s.curBlock
 	b.Kind = ssa.BlockJumpTable