[cleanup][turbofan] Move common SmiUntag code out of if else body

Bug: v8:10021
Change-Id: Id0616fa2c320bd5a817745310debf63fcf58e107
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1950490
Auto-Submit: Santiago Aboy Solanes <solanes@chromium.org>
Reviewed-by: Tobias Tebbi <tebbi@chromium.org>
Commit-Queue: Tobias Tebbi <tebbi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#65348}

src/codegen/x64/macro-assembler-x64.cc

@@ -1117,11 +1117,10 @@ void TurboAssembler::SmiUntag(Register dst, Operand src) {
     DCHECK(SmiValuesAre31Bits());
     if (COMPRESS_POINTERS_BOOL) {
       movsxlq(dst, src);
-      sarq(dst, Immediate(kSmiShift));
     } else {
       movq(dst, src);
-      sarq(dst, Immediate(kSmiShift));
     }
+    sarq(dst, Immediate(kSmiShift));
   }
 }