Subzero: Fix frame size for floating-point register out-args.

Subzero: Fix frame size for floating-point register out-args.

The code that calculates maximum out-arg stack space was neglecting the fact that on x86-64, the first N scalar floating-point arguments are passed through xmm registers, not the stack.  As a result, stack frames were sometimes larger than necessary.

BUG= none
R=jpp@chromium.org

Review URL: https://codereview.chromium.org/2076663006 .