Use an IRBuilder to insert instructions in preWriteTileCfg().
While here, also remove some unnecessary bool return values.
There are some test changes because the IRBuilder folds
"trunc i16 8 to i8" to "i8 8", and that has knock-on effects on
instruction naming.
I ran into this when converting tests to opaque pointers and
noticed that this pass introduces unnecessary "bitcast ptr to ptr"
instructions.
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