shader: fix conversion for return type of various bit operations

findLSB, findMSB, and countBits always return a signed integer

Fixes:

    dEQP-GLES31.functional.shaders.builtin_functions.uniform.findLSBMinusOne.*
    dEQP-GLES31.functional.shaders.builtin_functions.integer.findmsb.*
    dEQP-GLES31.functional.shaders.builtin_functions.integer.bitcount.*

    Changes resulting from the discussion on mesa-dev with
    Roland Schneidegger:
    v2: Set values in infer_(src|dst)_type only
    v3: Set values in the common infer_type code path, and only add
        the source type for UMSB

Signed-off-by: Gert Wollny <gert.wollny@collabora.com>
Reviewed-by: Dave Airlie <airlied@redhat.com>