i965/fs: Add (sub)reg_offset asserts to brw_reg_from_fs_reg.

These are completely ignored by the conversion to brw_reg, so they
better be zero.

Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>

src/mesa/drivers/dri/i965/brw_fs_generator.cpp

@@ -56,6 +56,7 @@ brw_file_from_reg(fs_reg *reg)
 static struct brw_reg
 brw_reg_from_fs_reg(fs_inst *inst, fs_reg *reg, unsigned gen, bool compressed)
 {
+   assert(reg->reg_offset == 0);
    struct brw_reg brw_reg;
 
    switch (reg->file) {
@@ -103,6 +104,7 @@ brw_reg_from_fs_reg(fs_inst *inst, fs_reg *reg, unsigned gen, bool compressed)
    case ARF:
    case FIXED_GRF:
    case IMM:
+      assert(reg->subreg_offset == 0);
       brw_reg = reg->as_brw_reg();
       break;
    case BAD_FILE: