PR80131: Simplification of 1U << (31 - x)

PR80131: Simplification of 1U << (31 - x)

Currently the code A << (B - C) is not simplified.
However at least a more specific case of 1U << (C -x) where
C = precision(type) - 1 can be simplified to (1 << C) >> x.

This is done by adding a new simplification rule in match.pd.

2017-11-07  Sudakshina Das  <sudi.das@arm.com>

    gcc/
	PR middle-end/80131
	* match.pd: Simplify 1 << (C - x) where C = precision (x) - 1.

    testsuite/
	PR middle-end/80131
	* testsuite/gcc.dg/pr80131-1.c: New Test.

git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@254496 138bc75d-0d04-0410-961f-82ee72b054a4