[Liftoff] Use int for liftoff register codes

The number of registers is very small, so any integer type is big
enough anyway. Just use the simplest one, which is int.

R=ahaas@chromium.org

Bug: v8:6600
Change-Id: I9762a5c934848b810b7937ee603cd183f883d557
Reviewed-on: https://chromium-review.googlesource.com/c/1394548Reviewed-by: Andreas Haas <ahaas@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#58524}

src/wasm/baseline/liftoff-register.h

@@ -153,7 +153,7 @@ class LiftoffRegister {
     return DoubleRegister::from_code(code_ - kAfterMaxLiftoffGpRegCode);
   }
 
-  uint32_t liftoff_code() const {
+  int liftoff_code() const {
     DCHECK(is_gp() || is_fp());
     return code_;
   }
@@ -265,13 +265,13 @@ class LiftoffRegList {
 
   LiftoffRegister GetFirstRegSet() const {
     DCHECK(!is_empty());
-    unsigned first_code = base::bits::CountTrailingZeros(regs_);
+    int first_code = base::bits::CountTrailingZeros(regs_);
     return LiftoffRegister::from_liftoff_code(first_code);
   }
 
   LiftoffRegister GetLastRegSet() const {
     DCHECK(!is_empty());
-    unsigned last_code =
+    int last_code =
         8 * sizeof(regs_) - 1 - base::bits::CountLeadingZeros(regs_);
     return LiftoffRegister::from_liftoff_code(last_code);
   }