[PowerPC] Materialize 34-bit constants with pli directly

Previously, 34-bit constants were materialized in selectI64Imm(), and we relied
on td pattern matching to instead produce a pli. This becomes problematic as
there is no guarantee that the 34-bit constant will reach the td pattern
selection for pli. It is also possible for other transformations (such as complex
bit permutations) to also produce and utilize the 34-bit constant materialized
through selectI64Imm().

This patch instead produces pli on Power10 directly whenever the constant fits
within 34-bits.

Differential Revision: https://reviews.llvm.org/D99906