[PowerPC] Make LDtocL and friends invariant loads

LDtocL, and other loads that roughly correspond to the TOC_ENTRY SDAG node,
represent loads from the TOC, which is invariant. As a result, these loads can
be hoisted out of loops, etc. In order to do this, we need to generate
GOT-style MMOs for TOC_ENTRY, which requires treating it as a legitimate memory
intrinsic node type. Once this is done, the MMO transfer is automatically
handled for TableGen-driven instruction selection, and for nodes generated
directly in PPCISelDAGToDAG, we need to transfer the MMOs manually.

Also, we were not transferring MMOs associated with pre-increment loads, so do
that too.

Lastly, this fixes an exposed bug where R30 was not added as a defined operand of
UpdateGBR.

This problem was highlighted by an example (used to generate the test case)
posted to llvmdev by Francois Pichet.

llvm-svn: 230553