This commit fixes `Frac(Float)` and `Frac(Float4)` generic LLVM code
generation.

See also: https://bugs.chromium.org/p/swiftshader/issues/detail?id=74

Bug: b/115344057
Test: functional.shaders.builtin_functions.precision.fract.highp_vertex
Test: functional.shaders.builtin_functions.precision.fract.highp_fragment
Change-Id: I027b7ab44ba3060dc100a220ba19e5275f41d4c9
Reviewed-on: https://swiftshader-review.googlesource.com/20933Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Chris Forbes <chrisforbes@google.com>
Tested-by: Chris Forbes <chrisforbes@google.com>