[PowerPC] Follow-up to r318436 to get the missed CSE opportunities
The last of the three patches that https://reviews.llvm.org/D40348 was
broken up into.
Canonicalize the materialization of constants so that they are more likely
to be CSE'd regardless of the bit-width of the use. If a constant can be
materialized using PPC::LI, materialize it the same way always.
li 4, -1 li 4, 255 li 4, 65535
are equivalent if the uses only use the low byte. Canonicalize it to the
Differential Revision: https://reviews.llvm.org/D40348