[mlir][llvm] Tablegen based operation import from LLVM IR.

[mlir][llvm] Tablegen based operation import from LLVM IR.

The revision uses tablegen generated builders to convert the most common
LLVM IR instructions to MLIR LLVM dialect operations. All instructions
with special handlers, except for alloca and fence, still use manual
handlers. The revision also introduces an additional "instructions.ll"
test file to test the import of instructions that have tablegen builders
(except for the resume instruction whose test remains untouched). A part
of the test cases are new, for example the integer instruction test,
while others are migrated from the "basic.ll" test file.

Reviewed By: ftynse

Differential Revision: https://reviews.llvm.org/D135709