MIPS[64]: Fix call to PrepareCallCFunction in liftoff

In liftoff, call to PrepareCallCFunction was using at as scratch
register. This is a very bad idea, since at is later used in
underlying macro assembler

TEST=cctest/test-run-wasm-64/RunWasmLiftoff_Regression_6858

Change-Id: Ifbc43678731d2833d2faa2f20fe79ea9e3089002
Reviewed-on: https://chromium-review.googlesource.com/1030430Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Sreten Kovacevic <sreten.kovacevic@mips.com>
Commit-Queue: Ivica Bogosavljevic <ivica.bogosavljevic@mips.com>
Cr-Commit-Position: refs/heads/master@{#52814}

src/wasm/baseline/mips/liftoff-assembler-mips.h

@@ -1252,9 +1252,8 @@ void LiftoffAssembler::CallC(wasm::FunctionSig* sig,
   mov(kFirstArgReg, sp);
 
   // Now call the C function.
-  constexpr Register kScratch = at;
   constexpr int kNumCCallArgs = 1;
-  PrepareCallCFunction(kNumCCallArgs, kScratch);
+  PrepareCallCFunction(kNumCCallArgs, kScratchReg);
   CallCFunction(ext_ref, kNumCCallArgs);
 
   // Move return value to the right register.

src/wasm/baseline/mips64/liftoff-assembler-mips64.h

@@ -1029,9 +1029,8 @@ void LiftoffAssembler::CallC(wasm::FunctionSig* sig,
   mov(kFirstArgReg, sp);
 
   // Now call the C function.
-  constexpr Register kScratch = at;
   constexpr int kNumCCallArgs = 1;
-  PrepareCallCFunction(kNumCCallArgs, kScratch);
+  PrepareCallCFunction(kNumCCallArgs, kScratchReg);
   CallCFunction(ext_ref, kNumCCallArgs);
 
   // Move return value to the right register.