Avoid ICE for nested inductions (PR 83914)

Avoid ICE for nested inductions (PR 83914)

This testcase ICEd because we converted the initial value of an
induction to the vector element type even for nested inductions.
This isn't necessary because the initial expression is vectorised
normally, and it meant that init_expr was no longer the original
statement operand by the time we called vect_get_vec_def_for_operand.

Also, adding the conversion code here made the existing SLP conversion
redundant.

2018-01-19  Richard Sandiford  <richard.sandiford@linaro.org>

gcc/
	PR tree-optimization/83914
	* tree-vect-loop.c (vectorizable_induction): Don't convert
	init_expr or apply the peeling adjustment for inductions
	that are nested within the vectorized loop.

gcc/testsuite/
	PR tree-optimization/83914
	* gcc.dg/vect/pr83914.c: New test.

git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@256884 138bc75d-0d04-0410-961f-82ee72b054a4