Add rules to strip away unneeded type casts in expressions

Add rules to strip away unneeded type casts in expressions

This patch moves part of the type conversion code from convert.c to match.pd
because match.pd is able to apply these transformations in the presence of
intermediate temporary variables.

Concretely it makes both these cases behave the same

  float e = (float)a * (float)b;
  *c = (_Float16)e;

and

  *c = (_Float16)((float)a * (float)b);

gcc/ChangeLog:

	* convert.c (convert_to_real_1): Move part of conversion code...
	* match.pd: ...To here.

gcc/testsuite/ChangeLog:

	* gcc.dg/type-convert-var.c: New test.

From-SVN: r273826