update_stmt calls

[Zdenek Dvorak <rakdver at atrey dot karlin dot mff dot cuni dot cz>]

Hello,

during a recent discussion, it was pointed to my attention that
update_stmt is performance critical.  I wondered why; this is the number
of update_stmt calls for combine.i (all the other passes have less then
1000 calls):

 tree VRP              :   17543 (10%)
 tree copy propagation :   3210 ( 2%)
 tree alias analysis   :   113845 (67%)
 tree SSA other        :   20824 (12%)
 dominator optimization:   1411 ( 1%)
 tree split crit edges :   3122 ( 2%)
 tree FRE              :   2060 ( 1%)
 expand                :   5703 ( 3%) 
 TOTAL                 :   170549

I have a patch that decreases number of update_stmt calls in tree alias
analysis to 46525; still, is it really that useful to run pass_may_alias
*six* times during compilation?  Obviously, we need the initial one, and
there are comments after pass_sra and pass_fold_builtins that indicate
that the following pass_may_alias cannot be avoided (which seems
doubtful to me, at the very least in the later case), but the remaining
three seem to be just placed randomly.

I also have a patch that decreases the number of update_stmt calls
in VRP to 5229 (which is more or less the number of ASSERT_EXPRs it
creates, so this cannot be improved significantly).

Zdenek