[test] Fix new CodeGenPrepare test for non-X86 systems

The new test llvm/test/Transforms/CodeGenPrepare/remove-assume-block.ll
breaks on non-X86 machines. Change it to look like the existing test
llvm/test/Transforms/CodeGenPrepare/X86/delete-assume-dead-code.ll
to fix it.

Reviewed By: bkramer

Differential Revision: https://reviews.llvm.org/D97952