Disable web, gcse-las, and gcse-sm by default

[Paolo Bonzini]

 > Amazingly (I don't know how to explain this yet) when the -fweb
> is used the scheduler knows that the loads are independent of the stores

Probably because the webizer understands the different live ranges and 
assigns the various loads to different pseudos.  Having a tree-level 
loop unroller will make this work again, because then -fweb's 
optimization will be performed during the out-of-ssa phase.

Paolo