[PDLL] Add a `rewrite` statement to enable complex rewrites

The `rewrite` statement allows for rewriting a given root
operation with a block of nested rewriters. The root operation is
not implicitly erased or replaced, and any transformations to it
must be expressed within the nested rewrite block. The inner body
may contain any number of other rewrite statements, variables, or
expressions.

Differential Revision: https://reviews.llvm.org/D115299