ARM NEON: Don't need COPY_TO_REGCLASS in pattern

In my previous commit:
"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."

I added a pattern containing a copy_to_regclass. The copy_to_regclass is
actually not needed.

radar://13191881