panfrost/midgard: Add shifting int modifiers

panfrost/midgard: Add shifting int modifiers

As a source modifier, shift allows shifting a value left by the bit
size, useful in conjunction with a greater register mode, for instance
to implement `upsample`. As a concrete example, the following OpenCL:

   ushort hr0 = /* ... */, uint r1 = /* ... */;
   uint r2 = (convert_uint(hr0) << 16) ^ b;

compiles to the following Midgard assembly:

   ixor r, (hr0) << 16, b

In reverse, the ".hi" output modifier shifts the value right by the bit
size, leaving just the carry/overflow at the bottom. To implement *_hi
functions in OpenCL (for <64-bit), we do arithmetic in the 2x higher
mode with the .hi modifier. (For 64-bit, things are hairier, since there
is not an 128-bit int mode).

Signed-off-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>