[MergeICmps] Fix build of rL312315 on clang-with-thin-lto-windows:

MergeICmps.cpp(68,15): error: chosen constructor is explicit in copy-initialization
      return {};
APInt.h(339,12): note: explicit constructor declared here
  explicit APInt() : BitWidth(1) { U.VAL = 0; }
             ^
MergeICmps.cpp(56,9): note: in implicit initialization of field 'Offset' with omitted
initializer
  APInt Offset;
          ^

llvm-svn: 312326