[PowerPC] define target hook isReallyTriviallyReMaterializable()

[PowerPC] define target hook isReallyTriviallyReMaterializable()

Define target hook isReallyTriviallyReMaterializable() to explicitly specify
PowerPC instructions that are trivially rematerializable.  This will allow
the MachineLICM pass to accurately identify PPC instructions that should always
be hoisted.

Differential Revision: https://reviews.llvm.org/D34255

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@305932 91177308-0d34-0410-b5e6-96231b3b80d8