[llvm] [PowerPC] Fix wrteei crash: use u1imm instead of i1imm (PR #188558)

[via llvm-commits]

Scottcjn wrote:

Rebased onto current main (dbf2384). Clean replay, no conflicts. The wrteei `u1imm` change and the ppc-wrteei-validate.s test are unchanged. Thanks @lei137.

https://github.com/llvm/llvm-project/pull/188558