Add ARM-specific DAG combining to cast i64 vector element load/stores to f64.

Type legalization splits up i64 values into pairs of i32 values, which leads
to poor quality code when inserting or extracting i64 vector elements.
If the vector element is loaded or stored, it can be treated as an f64 value
and loaded or stored directly from a VPR register.  Use the pre-legalization
DAG combiner to cast those vector elements to f64 types so that the type
legalizer won't mess them up.  Radar 8755338.

llvm-svn: 122319