R600/SI: Rename SOPP operands to match the encoding fields
R600/SI: Use a custom encoding method for simm16 in SOPP branch instructions

This allows us to explicitly define the type of fixup that is needed,
so we can distinguish this from future fixup types.
R600/SI: Use VALU for i1 XOR
R600/SI: Add isCFDepth0 Predicate to SALU addc pattern
R600/SI: Store constant initializer data in constant memory

This implements a solution for constant initializers suggested
by Vadim Girlin, where we store the data after the shader code
and then use the S_GETPC instruction to compute its address.

This saves use the trouble of creating a new buffer for constant data
and then having to pass the pointer to the kernel via user SGPRs or the
input buffer.