[DAGCombine] Add simple folds for SSHLSAT/USHLSAT

Do "simplifyShift" and "FoldConstantArithmetic" folds for the SSHLSAT
and USHLSAT DAG nodes.

This includes folds such as:
  (shlsat undef/poison, x) -> 0
  (shlsat x, undef/poison) -> undef
  (shlsat x, too_large_shamt) -> undef
  (shlsat 0, x) -> 0
  (shlsat x, 0) -> x
  (shlsat c1, c2) -> c3

Differential Revision: https://reviews.llvm.org/D118603