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 .

tests_lit/llvm2ice_tests/bitcast.ll

diff --git a/tests_lit/llvm2ice_tests/bitcast.ll b/tests_lit/llvm2ice_tests/bitcast.ll
index b5e9066..399d3fe 100644
--- a/tests_lit/llvm2ice_tests/bitcast.ll
+++ b/tests_lit/llvm2ice_tests/bitcast.ll
@@ -19,7 +19,7 @@
   ret i32 %v0
 }
 ; CHECK-LABEL: cast_f2i
-; CHECK: mov eax
+; CHECK: movd eax
 ; ARM32-LABEL: cast_f2i
 ; ARM32: vmov r{{[0-9]+}}, s{{[0-9]+}}