[AMDGPU] Added target-specific attribute amdgpu-max-memory-clause

With LLPC, previous investigation has suggested that si-scheduler
interacts badly with SiFormMemoryClauses on an XNACK target in some
games.

That needs further investigation in the future. In the meantime, this
commit adds a target-specific attribute to allow us to disable
SIFormMemoryClauses by setting it to 1 on a per-function basis for LLPC
to use.

Differential Revision: https://reviews.llvm.org/D62572

Change-Id: Ia0ca12ce79093cbbe86caded723ffb13384ede92
llvm-svn: 362127