Move LLVMIR dialect tests from test/LLVMIR to test/Dialect and test/Conversion

This follows up on the recent restructuring that moved the dialects under
lib/Dialect and inter-dialect conversions to lib/Conversion. Originally, the
tests for both the LLVMIR dialect itself and the conversion from Standard to
LLVMIR dialect lived under test/LLVMIR.  This no longer reflects the code
structure.  Move the tests to either test/Dialect/LLVMIR or
test/Conversion/StandardToLLVM depending on the features they exercise.

PiperOrigin-RevId: 267159219

Author: ftynse