compiler: use simpleOptExpr instead of simplifyExpr in hscCompileCoreExpr

This MR makes hscCompileCoreExpr use simpleOptExpr instead of simplifyExpr, so to make it faster by avoiding the full Core pipeline when compiling TH splices to bytecode. How much improvement is yet to be measured, though at least it settles another TODO in the tree with a long history. This takes more work than merely switching to simpleOptExpr though, given the bytecode backend doesn't properly handle seq a b which used to be desugared in the Core pipeline, which has also been handled in a commit here.