Subzero: Fix compiler warnings.

src/IceTargetLoweringX86BaseImpl.h:6093:13: error: unused variable 'Src1RM' [-Werror,-Wunused-variable]
      auto *Src1RM = legalize(Src1, Legal_Reg | Legal_Mem);
            ^

src/IceTargetLoweringX86BaseImpl.h:4007:3: error: default label in switch which covers all enumeration values [-Werror,-Wcovered-switch-default]
  default:
  ^

src/IceTargetLoweringMIPS32.cpp:4065:3: error: default label in switch which covers all enumeration values [-Werror,-Wcovered-switch-default]
  default:
  ^

src/IceTargetLoweringARM32.cpp:4975:3: error: default label in switch which covers all enumeration values [-Werror,-Wcovered-switch-default]
  default:
  ^

BUG= none
R=capn@chromium.org

Review URL: https://codereview.chromium.org/2434643002 .
3 files changed