nir: Differentiate between signed and unsigned versions of find_msb

nir: Differentiate between signed and unsigned versions of find_msb

We also make the return types match GLSL.  The GLSL spec specifies that
findMSB and findLSB return a signed integer.  Previously, nir had them
return unsigned.  This updates nir's behavior to match what GLSL expects.

We also update the nir-to-fs generator to take the new instructions.  While
we're at it, we fix the case where the input to findMSB is zero.

Reviewed-by: Connor Abbott <cwabbott0@gmail.com>