i965/fs_live_variables: Use var_from_vgrf insead of repeating the calculation

Signed-off-by: Jason Ekstrand <jason.ekstrand@intel.com>
Reviewed-by: Matt Turner <mattst88@gmail.com>

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

@@ -56,7 +56,7 @@ void
 fs_live_variables::setup_one_read(bblock_t *block, fs_inst *inst,
                                   int ip, fs_reg reg)
 {
-   int var = var_from_vgrf[reg.reg] + reg.reg_offset;
+   int var = var_from_reg(&reg);
    assert(var < num_vars);
 
    /* In most cases, a register can be written over safely by the
@@ -108,7 +108,7 @@ void
 fs_live_variables::setup_one_write(bblock_t *block, fs_inst *inst,
                                    int ip, fs_reg reg)
 {
-   int var = var_from_vgrf[reg.reg] + reg.reg_offset;
+   int var = var_from_reg(&reg);
    assert(var < num_vars);
 
    start[var] = MIN2(start[var], ip);