MIPS: Tweak Math.exp.

Port r16913 (a1b66fea)

Original commit message:
Avoid corrupting the input and small assembly tuning.

BUG=
R=plind44@gmail.com

Review URL: https://codereview.chromium.org/24654002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@16951 ce2b1a6d-e550-0410-aec6-3dcde31c8c00