Reimplement LLVM IR translation to use the MLIR LLVM IR dialect

Original implementation of the translation from MLIR to LLVM IR operated on the
Standard+BuiltIn dialect, with a later addition of the SuperVector dialect.
This required the translation to be aware of a potetially large number of other
dialects as the infrastructure extended.  With the recent introduction of the
LLVM IR dialect into MLIR, the translation can be switched to only translate
the LLVM IR dialect, and the translation of the operations becomes largely
mechanical.

The reimplementation of the translator follows the lines of the original
translator in function and basic block conversion.  In particular, block
arguments are converted to LLVM IR PHI nodes, which are connected to their
sources after all blocks of a function had been converted.  Thanks to LLVM IR
types being wrapped in the MLIR LLVM dialect type, type conversion is
simplified to only convert function types, all other types are simply
unwrapped.  Individual instructions are constructed using the LLVM IRBuilder,
which has a great potential for being table-generated from the LLVM IR dialect
operation definitions.

The input of the test/Target/llvmir.mlir is updated to use the MLIR LLVM IR
dialect.  While it is now redundant with the dialect conversion test, the point
of the exercise is to guarantee exactly the same LLVM IR is emitted.  (Only the
name of the allocation function is changed from `__mlir_alloc` to `alloc` in
the CHECK lines.)  It will be simplified in a follow-up commit.

PiperOrigin-RevId: 233842306