This commit fixes Floor() generic LLVM code generation.

Bug: b/115344057
Test: dEQP-GLES3.functional.shaders.builtin_functions.precision.floor
Change-Id: I70ec0babfe9778b7963296734f1901a73b1f696f
Reviewed-on: https://swiftshader-review.googlesource.com/20929Tested-by: Chris Forbes <chrisforbes@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Chris Forbes <chrisforbes@google.com>