This patch tries to clarify some of the restrictions on define_insn
conditions, and also on the use of "#".

2017-06-06  Richard Sandiford  <richard.sandiford@linaro.org>

gcc/
	* doc/md.texi: Clarify the restrictions on a define_insn condition.
	Say that # requires an associated define_split to exist, and that
	the define_split must be suitable for use after register allocation.

From-SVN: r248946