[PATCH] Make bsi_after_labels return a BSI pointing after the labels (4.1 too?)

[Paolo Bonzini <paolo dot bonzini at lu dot unisi dot ch>]

Right now, bsi_after_labels returns a BSI pointing to the last label, so 
that inserting with bsi_insert_after inserts a statement at the 
beginning of the block.

Originally bsi_after_labels aborted if the block had no labels, because 
then a bsi_insert_after would insert a statement as *second* and not 
*first*.  This changed with the patch PR23625, but it still can cause 
ice-on-valid errors with checking enabled (in case the second statement 
defines an SSA_NAME used in the first) and wrong-code errors with 
checking disabled.  I don't have a testcase on unpatched mainline, but 
it happened in a pass I'm not contributing to FSF GCC.

The solution is to make bsi_after_labels do what it's supposed to do, 
and change the users to use bsi_insert_before.  Only one user turned out 
to be a bit problematic, because we have to contextually change 
BSI_NEW_STMT to BSI_SAME_STMT, and scev_const_prop wants to call 
force_gimple_operand_bsi and pass a bsi for the newly inserted 
statement.  I fixed this by working on a copy of the BSI.

Bootstrapped/regtested i686-pc-linux-gnu, all languages.  Ok for 
mainline, and maybe for 4.1 after a week or less?

Paolo