i965/fs: disallow type change in copy-propagation if types have different sizes

i965/fs: disallow type change in copy-propagation if types have different sizes

Because the semantics of source modifiers are type-dependent, the type of the
original source of the copy must be kept unmodified while propagating it into
some instruction, which implies that we need to have the guarantee that the
meaning of the instruction is going to remain the same after we have changed
the types. Whenthe size of the new type is different from the size of the old
type the new and old instructions cannot possibly be equivalent because the new
instruction will be reading more data than the old one was.

Prevents that we turn this:

load_payload(8) vgrf17:DF, |vgrf4+0.0|:DF 1sthalf
mov(8) vgrf18:DF, vgrf17:DF 1sthalf
load_payload(8) vgrf5:DF, vgrf18:DF, vgrf20:DF NoMask 1sthalf WE_all
load_payload(8) vgrf21:UD, vgrf5+0.4<2>:UD 1sthalf
mov(8) vgrf22:UD, vgrf21:UD 1sthalf

into:

load_payload(8) vgrf17:DF, |vgrf4+0.0|:DF 1sthalf
mov(8) vgrf18:DF, |vgrf4+0.0|:DF 1sthalf
load_payload(8) vgrf5:DF, |vgrf4+0.0|:DF, |vgrf4+2.0|:DF NoMask 1sthalf WE_all
load_payload(8) vgrf21:UD, vgrf5+0.4<2>:UD 1sthalf
mov(8) vgrf22:DF, |vgrf4+0.4|<2>:DF 1sthalf

where the semantics of the last instruccion have changed.

v2 (Curro):
  - Update commit log and add comment to explain the problem better.
  - Simplify the condition.

Reviewed-by: Francisco Jerez <currojerez@riseup.net>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>