[X86PreAMXConfig] Use IRBuilder to insert instructions (NFC)

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.