MIPS64: Fix RegExpExecStub::Generate().

TEST: mjsunit/regress/regress-227
MIPS64: Fix RegExpMacroAssemblerMIPS::GetCode().

This commit can fix part of mjsunit/regress/regress-176.
MIPS64: Fix load result from regexp data.

TEST: mjsunit/regress/regress-176
MIPS64: Fix style issue, add some TODO's.
MIPS64: Fix for incremental marking.

I believe the code is OK now, but it really needs better comments to
explain some of the insanity.