[RISCV] Use f64 for LocVT for ilp32 when whole f64 is passed on the stack. NFC (#69118)

This removes the special case from unpackF64OnRV32DSoftABI. We can use
the default MemLoc handling.

This also allows us to remove a isRegLoc() check from LowerCall.

This part of preparation for supporting FP arguments with GISel.