MIPS32: Use conditional moves to compute 64-bit shifts.

Use conditional moves in
InstructionCodeGeneratorMIPS::HandleShift()'s 64-bit variable
shifts to avoid conditional branches (Beqz(TMP, &done)).

Also, on R6 use Beqzc(TMP, &done, /* is_bare */ true) in place of
Beqz(TMP, &done).

Test: Boot & run tests on MIPS32r6 QEMU & on CI-20 hardware (MIPS32r2).
Test: test/testrunner/testrunner.py --target --optimizing

Change-Id: I4d34a51cd2397c845f936af853cb5f30e82de438