AMDGPU/SI: Add new target attribute InitialPSInputAddr

AMDGPU/SI: Add new target attribute InitialPSInputAddr

Summary:
This allows Mesa to pass initial SPI_PS_INPUT_ADDR to LLVM.
The register assigns VGPR locations to PS inputs, while the ENA register
determines whether or not they are loaded.

Mesa needs to set some inputs as not-movable, so that a pixel shader prolog
binary appended at the beginning can assume where some inputs are.

v2: Make PSInputAddr private, because there is never enough silly getters
    and setters for people to read.

Reviewers: tstellarAMD, arsenm

Subscribers: arsenm

Differential Revision: http://reviews.llvm.org/D16030

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@257591 91177308-0d34-0410-b5e6-96231b3b80d8