Use gimple_build to perform conversion simplification

Use gimple_build to perform conversion simplification

The following uses gimple_build to do the conversion simplification
in build_and_insert_cast instead of duplicating it there.  Conveniently
when building directly into the IL all stmts are taken into account
for the simplification.

	PR tree-optimization/122111
	* tree-ssa-math-opts.cc (build_and_insert_cast): Remove
	conversion simplification, instead use gimple_convert.

	* gcc.target/arm/pr122111.c: New test.