[FastISel] Fix a bug in FastISel::CallLoweringInfo.

This fixes a bug in FastISel::CallLoweringInfo, where the number of
arguments was obtained from the argument vector before it had been
initialized.

Test case follows in another commit.
[FastISel][AArch64] Add lowering support for frem.

This lowers frem to a runtime libcall inside fast-isel.

The test case also checks the CallLoweringInfo bug that was exposed by this
change.

This fixes rdar://problem/18342783.