Do not use doloop pattern with pragma Unroll

Do not use doloop pattern with pragma Unroll

This fixes the following Ada failure on 64-bit PowerPC:

-FAIL: gnat.dg/unroll4.adb scan-rtl-dump-times loop2_unroll "optimized:
loop unrolled 7 times" 2

The IVOPTS pass detects a doloop pattern and consequently discombobulates
the loop sufficiently as to make it hard for the RTL unrolling pass to
compute the best number of iterations.

gcc/ChangeLog:
	* tree-ssa-loop-ivopts.c (analyze_and_mark_doloop_use): Bail out if
	the loop is subject to a pragma Unroll with no specific count.