intel/brw: Drop misguided sign extension attempts in extract_imm()

This function never expands a type - it only narrows it.  As such, we
don't need to ever sign extend to fill additional new bits.  I think
this code was left over from earlier versions of my optimization pass
that was buggy and trying to handle cases it should not have.

Fixes: 580e1c592d ("intel/brw: Introduce a new SSA-based copy propagation pass")
Reviewed-by: Caio Oliveira <caio.oliveira@intel.com>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/30884>

src/intel/compiler/brw_fs_copy_propagation.cpp

@@ -1740,16 +1740,7 @@ extract_imm(brw_reg val, brw_reg_type type, unsigned offset)
 
    assert(bitsize < brw_type_size_bits(val.type));
 
-   switch (val.type) {
-   case BRW_TYPE_UD:
-      val.ud = (val.ud >> (bitsize * offset)) & ((1u << bitsize) - 1);
-      break;
-   case BRW_TYPE_D:
-      val.d = (val.d << (bitsize * (32/bitsize - 1 - offset))) >> ((32/bitsize - 1) * bitsize);
-      break;
-   default:
-      return brw_reg();
-   }
+   val.ud = (val.ud >> (bitsize * offset)) & ((1u << bitsize) - 1);
 
    return val;
 }