Reactor: Fix Trunc generic code generation

Reactor: Fix Trunc generic code generation

This commit fixes `Trunc(Float)` and `Trunc(Float4)` generic LLVM code
generation.  If `Trunc(x)` is implemented with `Float(Int(x))`, it will
result in quality warning in dEQP.

Bug: b/115344057
Test: dEQP-GLES3.functional.shaders.builtin_functions.precision.trunc
Test: dEQP-GLES3.functional.shaders.builtin_functions.precision.modf
Change-Id: I62d2dd1907e345fb00307b6c0d4d74613237f94b
Reviewed-on: https://swiftshader-review.googlesource.com/21029
Tested-by: Chris Forbes <chrisforbes@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Chris Forbes <chrisforbes@google.com>