i965/fs: Set execution controls correctly for lowered pull constant loads.

i965/fs: Set execution controls correctly for lowered pull constant loads.

demote_pull_constants() was ignoring the execution size and channel
selects of the instruction that wanted the constant, which doesn't
matter for uniform pull constant loads because all channels get the
same scalar value, but it might for varying pull constant loads.  Fix
it by using the new fs_builder() constructor that takes care of
setting execution controls compatible with the instruction passed as
argument.

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