MIPS: [turbofan] Optimize sign-extension patterns like Sar(Shl(x, a), b)).

src/compiler/mips/code-generator-mips.cc

Index: src/compiler/mips/code-generator-mips.cc
diff --git a/src/compiler/mips/code-generator-mips.cc b/src/compiler/mips/code-generator-mips.cc
index 61d57bc46e8cae6a0ddc4669b99f71d3e48220c6..90fe9e7cf595657290c11d176c997df6a5150641 100644
--- a/src/compiler/mips/code-generator-mips.cc
+++ b/src/compiler/mips/code-generator-mips.cc
@@ -1355,7 +1355,12 @@ CodeGenerator::CodeGenResult CodeGenerator::AssembleArchInstruction(
       break;
 
     // ... more basic instructions ...
-
+    case kMipsSeb:
+      __ seb(i.OutputRegister(), i.InputRegister(0));
+      break;
+    case kMipsSeh:
+      __ seh(i.OutputRegister(), i.InputRegister(0));
+      break;
     case kMipsLbu:
       __ lbu(i.OutputRegister(), i.MemoryOperand());
       break;