i965/fs: Make the default builder 64-wide before entering the optimization loop.

i965/fs: Make the default builder 64-wide before entering the optimization loop.

Not a typo.  Replace the default builder with one of bogus width to
catch cases in which optimization passes assume that the default
dispatch width is good enough.  The execution controls of instructions
emitted during optimization should in general match the original code
that is being manipulated.  Many of the problems fixed in this series
were caught by the assertions introduced in this patch.

Reviewed-by: Jason Ekstrand <jason.ekstrand@intel.com>