intel/compiler: Fix assertions in brw_alu3

v2: Fix assertion for src1 (Ian Romanick)

Fixes: 3b967e17 (intel/compiler: Avoid false positive assertions)
Signed-off-by: Sagar Ghuge <sagar.ghuge@intel.com>
Suggested-by: Matt Turner <mattst88@gmail.com>
Reviewed-by: Matt Turner <mattst88@gmail.com>
10 jobs for !697 with 3imm in 9 minutes and 1 second (queued for 2 seconds)
latest detached
Status Job ID Name Coverage
  Containers Build
passed debian #334982

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  Build+Test
passed meson-clang #334984

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passed meson-clover #334988

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passed meson-main #334987

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passed meson-swr-glvnd #334983

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passed meson-vulkan #334989

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passed scons-llvm #334991

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passed scons-nollvm #334990

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passed scons-swr #334985

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passed scons-win64 #334986

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