Gallium/tgsi: Correct signdness of return value of bit operations

Gallium/tgsi: Correct signdness of return value of bit operations

The GLSL operations findLSB, findMSB, and countBits always return
a signed integer type. Let TGSI reflect this.

v2: Properly set values in infer_(src|dst)_type   (Thanks Roland
    Schneidegger for pointing out problems with my 1st approach)
v2: Set values in the common infer_type code path, and only add
    the correct source type for UMSB (Roland Schneidegger)

Signed-off-by: Gert Wollny <gert.wollny@collabora.com>
Reviewed-by: Roland Scheidegger <sroland@vmware.com>