gallium/tgsi: Treat UCMP sources as floats to match the GLSL-to-TGSI pass expectations.

Currently the GLSL-to-TGSI translation pass assumes it can use
floating point source modifiers on the UCMP instruction.  See the bug
report linked below for an example where an unrelated change in the
GLSL built-in lowering code for atan2 (e9ffd12827ac11a2d2002a42fa8eb1)
caused the generation of floating-point ir_unop_neg instructions
followed by ir_triop_csel, which is translated into UCMP with a negate
modifier on back-ends with native integer support.

Allowing floating-point source modifiers on an integer instruction
seems like rather dubious design for a transport IR, since the same
semantics could be represented as a sequence of MOV+UCMP instructions
instead, but supposedly this matches the expectations of TGSI
back-ends other than tgsi_exec, and the expectations of the DX10 API.
I take no responsibility for future headaches caused by this
inconsistency.

Fixes a regression of piglit glsl-fs-tan-1 on softpipe introduced by
the above-mentioned glsl front-end commit.  Even though the commit
that triggered the regression doesn't seem to have made it to any
stable branches yet, this might be worth back-porting since I don't
see any reason why the bug couldn't have been reproduced before that
point.

Suggested-by: Roland Scheidegger <[email protected]>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=99817
Reviewed-by: Roland Scheidegger <[email protected]>

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