These are identified by misc-include-cleaner.  I've filtered out those
that break builds.  Also, I'm staying away from llvm-config.h,
config.h, and Compiler.h, which likely cause platform- or
compiler-specific build failures.

Add a rewriter for DIExpressions & use it to run legalization patterns
before exporting to llvm (because LLVM dialect allows DI Expressions
that may not be valid in LLVM IR).

The rewriter driver works similarly to the existing mlir rewriter
drivers, except it operates on lists of DIExpressionElemAttr (i.e.
DIExpressionAttr). Each rewrite pattern transforms a range of
DIExpressionElemAttr into a new list of DIExpressionElemAttr.

In addition, this PR sets up a place to add legalization patterns that
are broadly applicable internally to the LLVM dialect, and they will
always be applied prior to export. This PR adds one pattern for merging
fragment operators.

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Co-authored-by: Tobias Gysi <tobias.gysi@nextsilicon.com>