MIPS: Fix clang build by removing incorrect inline asm constraint.

Improper use of immediate constraint is slightly non-optimal
with gcc/gnu-as but causes breakage with clang.

This is a cherry-pick of upstream v8 bleeding_edge:
https://chromium.googlesource.com/v8/v8/+/eb68d6fe13f13b0697dd2b4ff00df1d7c997669e

BUG:18789639

Change-Id: I8786262b4bf906aec982b790cbf3982601980cf6
diff --git a/src/base/atomicops_internals_mips64_gcc.h b/src/base/atomicops_internals_mips64_gcc.h
index 1f629b6..ccb973c 100644
--- a/src/base/atomicops_internals_mips64_gcc.h
+++ b/src/base/atomicops_internals_mips64_gcc.h
@@ -59,7 +59,7 @@
                        "2:\n"
                        ".set pop\n"
                        : "=&r" (prev), "=m" (*ptr), "=&r" (tmp)
-                       : "Ir" (old_value), "r" (new_value), "m" (*ptr)
+                       : "r" (old_value), "r" (new_value), "m" (*ptr)
                        : "memory");
   return prev;
 }
@@ -196,7 +196,7 @@
                        "2:\n"
                        ".set pop\n"
                        : "=&r" (prev), "=m" (*ptr), "=&r" (tmp)
-                       : "Ir" (old_value), "r" (new_value), "m" (*ptr)
+                       : "r" (old_value), "r" (new_value), "m" (*ptr)
                        : "memory");
   return prev;
 }