[CMake] -gen-dag-isel: add -omit-comments if neither Debug nor RelWithDebInfo

Omitting comments can make the output much smaller. Size/time impact on
my machine:

* lib/Target/AArch64/AArch64GenDAGISel.inc, 10MiB (8.89s) -> 5MiB (3.20s)
* lib/Target/X86/X86GenDAGISel.inc, 20MiB (6.48s) -> 8.5MiB (4.18s)

In total, this change decreases lib/Target/*/*GenDAGISel.inc from
71.4MiB to 30.1MiB.

As rnk suggested, we can consider an option next to LLVM_OPTIMIZED_TABLEGEN
once we have more needs like this.

Reviewed By: thakis

Differential Revision: https://reviews.llvm.org/D78884