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

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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@173615 91177308-0d34-0410-b5e6-96231b3b80d8