This patch checks the type of the lhs operand of a floating point
comparison, and commutes the operands if it is #0.0.  It allows us to
optimize a comparison with zero, as the fcmp instruction accepts #0.0 as
rhs operand.

Code before for "0.0 < 0.123":
------------------------------
fmov d1, xzr
ldr d0, pc+96
fcmp d1, d0
b.lo #+0xc

Code after:
-----------
ldr d0, pc+92
fcmp d0, #0.0
b.gt #+0xc

Before this patch, we used unsigned condition codes for floating point
comparisons, but the unordered case was not correctly commuted.

Review URL: https://codereview.chromium.org/1356283003

Cr-Commit-Position: refs/heads/master@{#30881}