[GCC][middle-end] Add rules to strip away unneeded type casts in expressions (2nd patch)

[Tamar Christina]

Hi All,

This is an updated version of my GCC-9 patch which moves part of the type conversion code
from convert.c to match.pd because match.pd is able to apply this transformation in the
presence of intermediate temporary variables.

The previous patch was only regtested on aarch64-none-linux-gnu and I hadn't done a
regression on x86_64-pc-linux-gnu only a bootstrap.  The previous patch was approved

here https://gcc.gnu.org/ml/gcc-patches/2018-12/msg00116.html
but before committing I ran a x86_64-pc-linux-gnu regtest to be sure and this
showed an issue with a DFP test. I Have fixed this by removing the offending convert.
The convert was just saying "keep the type as is" but match.pd looped here as it thinks
the match did something and would try other patterns, causing it to match itself again.

Instead when there's nothing to update, I just don't do anything.

The second change was to merge this with the existing pattern for integer conversion
in order to silence a warning from match.pd which though that the two patterns overlaps
because their match conditions are similar (they have different conditions inside the ifs
but match.pd doesn't check those of course.).

Regtested and bootstrapped on aarch64-none-linux-gnu and x86_64-pc-linux-gnu and no issues.

Ok for trunk?

Thanks,
Tamar

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);

Thanks,
Tamar

gcc/ChangeLog:

2019-06-25  Tamar Christina  <tamar.christina@arm.com>

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

gcc/testsuite/ChangeLog:

2019-06-25  Tamar Christina  <tamar.christina@arm.com>

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

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