[MLIR] Create folders for extsi/extui

Create folders/canonicalizers for extsi/extui. Specifically,

extui(extui(x)) -> extui(x)
extsi(extsi(x)) -> extsi(x)
extsi(extui(x)) -> extui(x)

Reviewed By: mehdi_amini

Differential Revision: https://reviews.llvm.org/D116515