MIPS: Adjust code after JALR to JAL optimization removal

Now that JALR to JAL optimization is removed, the value of the constant
kInstructionsFor32BitConstant and comments are adjusted accordingly.

BUG=

Review URL: https://codereview.chromium.org/1690133004

Cr-Commit-Position: refs/heads/master@{#34235}

src/mips/assembler-mips.h

@@ -520,14 +520,11 @@ class Assembler : public AssemblerBase {
   // a target is resolved and written.
   static const int kSpecialTargetSize = 0;
 
-  // Number of consecutive instructions used to store 32bit constant.
-  // Before jump-optimizations, this constant was used in
-  // RelocInfo::target_address_address() function to tell serializer address of
-  // the instruction that follows LUI/ORI instruction pair. Now, with new jump
-  // optimization, where jump-through-register instruction that usually
-  // follows LUI/ORI pair is substituted with J/JAL, this constant equals
-  // to 3 instructions (LUI+ORI+J/JAL/JR/JALR).
-  static const int kInstructionsFor32BitConstant = 3;
+  // Number of consecutive instructions used to store 32bit constant. This
+  // constant is used in RelocInfo::target_address_address() function to tell
+  // serializer address of the instruction that follows LUI/ORI instruction
+  // pair.
+  static const int kInstructionsFor32BitConstant = 2;
 
   // Distance between the instruction referring to the address of the call
   // target and the return address.