R600: improve inputs/interpolation handling

Use one intrinsic for all sorts of interpolation.
Use two separate unexpanded instructions to represent INTERP_XY and _ZW -
this will allow to eliminate one part if it's not used.
Track liveness of special interpolation regs instead of reserving them -
this will allow to reuse those regs, lowering reg pressure.

Patch By: Vadim Girlin

v2[Vincent Lejeune]: Rebased against current llvm master

Signed-off-by: Vadim Girlin <vadimgirlin@gmail.com>
Reviewed-by: Tom Stellard <thomas.stellard@amd.com>
llvm-svn: 174394