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
84313162bf