MIPS: Fix and simplify code aging.

This commit fixes a lot of test failures that we saw earlier in the buildbots (http://build.chromium.org/p/client.v8/builders/V8%20Linux%20-%20mips%20-%20sim/builds/3034/steps/Check/logs/stdio).

In some very rare cases the code age stub address can be 0xXXXX0000 and in this case the li maco instruction emits only 1 instruction (instead of the expected 2). Thus the code age sequence will be 6 instructions long instead of 7, which breaks the code aging feature. This change makes sure that li always emits 2 instructions and it also simplifies the code aging sequence.

Also fixes a small mistake in the simulator at the jalr instruction.

BUG=
R=gergely@homejinni.com

Review URL: https://codereview.chromium.org/83583003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@18030 ce2b1a6d-e550-0410-aec6-3dcde31c8c00