ARM NEON: Merge a f32 bitcast of a v2i32 extractelt

A vectorized sitfp on doubles will get scalarized to a sequence of an
extract_element of <2 x i32>, a bitcast to f32 and a sitofp.
Due to the the extract_element, and the bitcast we will uneccessarily generate
moves between scalar and vector registers.

The patch fixes this by using a COPY_TO_REGCLASS and a EXTRACT_SUBREG to extract
the element from the vector instead.

radar://13191881

llvm-svn: 175520