[AMDGPU] Stop replacing amdgcn.ballot(1) with amdgcn.s.getreg(exec)

Rationale:
- It does not enable any further IR simplifications.
- It does not improve the generated code since the isel lowering of
  ballot also has special cases for 0 and 1.
- getreg is "too powerful" since it can read from many different
  registers, so its intrinsic properties have to be set very
  conservatively.

There is also a correctness problem that getreg can read from exec but
it is currently not marked as convergent.

Differential Revision: https://reviews.llvm.org/D153047