This gives very poor coverage. The testcases in one compiler's regressions suites are usually reduced to the shortest program that causes a previous bug, and another compiler that does not share any source code with the first one will have no particular reason to contain bugs that are revealed by the same programs. Sure, you may find some this way, but it is not likely to be the best use of your time. Invent something like Csmith/EMI instead, that will be more efficient!
The C language needs a good test suite with known expected results and good coverage of the language's definition, but one way to build this suite is not to catenate all the regression tests of existing compilers. Actually, part of the reasons why we do not have such a reference testsuite is that it is not as easy as putting together regression tests from various origins.