PPC: check input to xxspltib is 1 byte long

xxspltib takes in an immediate value of up to 8 bits.
We need to make sure input values are not bigger in size.

Change-Id: I3574028d43752870564e4690b22bf2e87813ab76
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2613031Reviewed-by: Junliang Yan <junyan@redhat.com>
Commit-Queue: Milad Fa <mfarazma@redhat.com>
Cr-Commit-Position: refs/heads/master@{#71930}

src/codegen/ppc/assembler-ppc.cc

@@ -1860,6 +1860,7 @@ void Assembler::stxvd(const Simd128Register rt, const MemOperand& dst) {
 
 void Assembler::xxspltib(const Simd128Register rt, const Operand& imm) {
   int TX = 1;
+  CHECK(is_uint8(imm.immediate()));
   emit(XXSPLTIB | rt.code() * B21 | imm.immediate() * B11 | TX);
 }