[AMDGPU] Fix clamp bit DAG operand

Summary:
- Should use `targetconstant` instead of `constant` operand for clamp
  bit, which is expected as an immediate operand. Under certain
  conditions, such as a common `i1 false` constant is used in other
  place and selected before the instruction with clamp bit, register
  operand may be added instead of immediate one. Use `targetcosntant` to
  enforce that.

Subscribers: arsenm, kzhuravl, jvesely, wdng, nhaehnle, yaxunl, dstuttard, tpr, t-tye, hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D59608

llvm-svn: 356608