regcomp.c: Consolidate 2nd pass for warnings

regcomp.c: Consolidate 2nd pass for warnings

Warnings have to generally be delayed being output until the 2nd pass,
as the first pass can be restarted multiple times, and so the same
warning could be output multiple times if the restarted code outputs a
warning.

Prior to this commit, there was an assert that the warnings are being
output in the 2nd pass.  This commit changes it so that the assert is
turned into an 'if' in common code, and the dispersed 'if's that
formerly were used are removed as much as possible.  If that removes an
indented block, this commit also outdents the block contents.