i965/compiler: handle conversion to smaller type in the lowering pass for that

i965/compiler: handle conversion to smaller type in the lowering pass for that

This rollbacks the revert of this same patch introduced in
commit 7b9c15628aae8729118b648f5f473e6ac926b99b.

And also squahes the following patch to prevent a piglit regression caused
by this change:

intel/compiler: Fix lower_conversions for 8-bit types.
Author: Jose Maria Casanova Crespo <jmcasanova@igalia.com>

For 8-bit types the execution type is word. A byte raw MOV has 16-bit
execution type and 8-bit destination and it shouldn't be considered
a conversion case. So there is no need to change alignment and enter
in lower_conversions for these instructions.

Fixes a regresion in the piglit test "glsl-fs-shader-stencil-export"
that is introduced with this patch from the Vulkan shaderInt16 series:
'i965/compiler: handle conversion to smaller type in the lowering
pass for that'. The problem is caused because there is already a case
in the driver that injects Byte instructions like this:

mov(8)          g127<1>UB       g2<32,8,4>UB

And the aforementioned pass was not accounting for the special
handling of the execution size of Byte instructions. This patch
fixes this.

v2: (Jason Ekstrand)
   - Simplify is_byte_raw_mov, include reference to PRM and not
   consider B <-> UB conversions as raw movs.

v3: (Matt Turner)
   - Indentation style fixes.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=106393
Tested-by: Mark Janes <mark.a.janes@intel.com>
Reviewed-by: Matt Turner <mattst88@gmail.com>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>