When the legalizer is splitting vector shifts, the result may not have the right shift amount type.

Fix that by adding a cast to the shift expander. This came up with vector shifts
on sse-less X86 CPUs.

   <2 x i64>       = shl <2 x i64> <2 x i64>
-> i64,i64         = shl i64 i64; shl i64 i64
-> i32,i32,i32,i32 = shl_parts i32 i32 i64; shl_parts i32 i32 i64

Now we cast the last two i64s to the right type. Fixes the crash in PR14668.