[SDAG] remove shift that is redundant with part of funnel shift

This is the SDAG translation of D120253 :
https://alive2.llvm.org/ce/z/qHpmNn

The SDAG nodes can have different operand types than the result value.
We can see an example of that with AArch64 - the funnel shift amount
is an i64 rather than i32.

We may need to make that match even more flexible to handle
post-legalization nodes, but I have not stepped into that yet.

Differential Revision: https://reviews.llvm.org/D120264