nir: do not assume that the result of fexp2(a) is always an integral

It's only correct when 'a' is an integral greater or equal to 0.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=111493
Fixes: 5544b2cb ("nir/algebraic: Use value range analysis to eliminate useless unary ops")
Signed-off-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
(cherry picked from commit 966a455b)
(conflicts resolved by Dylan Baker)
23 jobs for staging/19.2 in 14 minutes and 14 seconds (queued for 3 seconds)
Status Name Job ID Coverage
  Containers Build
passed debian #570279

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  Build+Test
passed meson-arm64 #570288

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passed meson-armhf #570287

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passed meson-clang #570281

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passed meson-clover #570285

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passed meson-i386 #570289

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passed meson-main #570284

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passed meson-swr-glvnd #570280

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passed meson-vulkan #570286

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passed scons-llvm #570291

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passed scons-nollvm #570290

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passed scons-swr #570282

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passed scons-win64 #570283

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  Test
passed test-llvmpipe-gles2 1/4 #570292

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passed test-llvmpipe-gles2 2/4 #570293

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passed test-llvmpipe-gles2 3/4 #570294

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passed test-llvmpipe-gles2 4/4 #570295

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passed test-softpipe-gles2 1/4 #570296

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passed test-softpipe-gles2 2/4 #570297

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passed test-softpipe-gles2 3/4 #570298

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passed test-softpipe-gles2 4/4 #570299

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passed test-softpipe-gles3-limited #570300

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  External
passed continuous-integration/appveyor/branch