[AMDGPU] Set number vgprs used in PS shaders based on input registers actually used

For PS shaders we can use the input SPI_PS_INPUT_ENA and SPI_PS_INPUT_ADDR
registers

Calculate the number of VGPR registers used as input VGPRs based on these
registers rather than the arguments passed in (this conservatively always
allocates the maximum).

Differential Revision: https://reviews.llvm.org/D101633

Change-Id: Idf7c060cbbd5f7e3300102c55ecee3c07f209de6