[mlir] Avoid creating local OpBuilders in Standard-to-LLVM conversion

Conversions of allocation-related operations in Standard-to-LLVM need
declarations of "malloc" and "free" (or equivalents). They use locally created
OpBuilders pointed at the module level to declare these functions if necessary.
This is poorly compatible with the pattern infrastructure that is unaware of
new operations being created. Update the insertion point of the main rewriter
instead.

Differential Revision: https://reviews.llvm.org/D82649