i965/fs: fix copy-propagation with suboffset from constants

The current code ignores the suboffet in the instruction's source
and just uses the one from the constant. This is not correct
when the instruction's source is accessing the constant with a
different type and using the suboffset to select a specific
chunk of the constant. We generate this kind of code in fp64
when we want to select only the high 32-bit of a particular
double constant.

Instead, we should add any existing suboffset in the
instruction's source (modulo the size of the entry's type)
to the suboffset in the constant so we can preserve the orinal
semantics.

Prevents that we turn this:

mov(8) vgrf5:DF, u2<0>:DF
mov(8) vgrf7:UD, vgrf5+0.4<2>:UD

Into:

mov(8) vgrf7:UD, u2<0>:UD

And instead, with this patch, we produce:

mov(8) vgrf7:UD, u2+0.4<0>:UD

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

@@ -445,8 +445,27 @@ fs_visitor::try_copy_propagate(fs_inst *inst, int arg, acp_entry *entry)
    case BAD_FILE:
    case ARF:
    case FIXED_GRF:
-      inst->src[arg].reg_offset = entry->src.reg_offset;
-      inst->src[arg].subreg_offset = entry->src.subreg_offset;
+      {
+         inst->src[arg].reg_offset = entry->src.reg_offset;
+         inst->src[arg].subreg_offset = entry->src.subreg_offset;
+
+         /* If we copy propagate from a larger type we have to be aware that
+          * the instruction might be using subreg_offset to select a particular
+          * chunk of the data in the entry. For example:
+          *
+          * mov(8) vgrf5:DF, u2<0>:DF
+          * mov(8) vgrf7:UD, vgrf5+0.4<2>:UD
+          *
+          * vgrf5+0.4<2>:UD is actually reading the high 32-bit of u2.0, so if
+          * we want to copy propagate here we have to do it from u2+0.4.
+          */
+         int type_sz_src = type_sz(inst->src[arg].type);
+         int type_sz_entry = type_sz(entry->src.type);
+         if (type_sz_entry > type_sz_src) {
+            inst->src[arg].subreg_offset +=
+               inst->src[arg].subreg_offset % type_sz_entry;
+         }
+      }
       break;
    case ATTR:
    case VGRF: