Subzero: Build a better bitcast.

The x86 lowering of bitcast between integers and floats forced the transfer through a stack slot (the original implementer *cough* *cough* wasn't aware of the movd instruction).  This requires excess instructions, but also a store to memory followed immediately by a load from that location is very slow.

This fixes the problem by using the movd instruction instead.

BUG= none
R=jpp@chromium.org

Review URL: https://codereview.chromium.org/2077503002 .