[turbofan] ARM64 FP instruction selection tests

Add some tests for floating pointer instruction selection, and change conversion
tests to be parameterised.

BUG=
R=bmeurer@chromium.org

Review URL: https://codereview.chromium.org/500503002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@23439 ce2b1a6d-e550-0410-aec6-3dcde31c8c00
Slightly simplify Math.sign and Math.trunc.

R=svenpanne@chromium.org

Review URL: https://codereview.chromium.org/504343005

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@23440 ce2b1a6d-e550-0410-aec6-3dcde31c8c00