This patch adds base 2 logarithm that returns integer result. I initially wanted to name it `!log2`,
but numbers are not permitted in the name. The documentation makes sure to clarify that it is
base 2 since it is not explicit in the operator name.
I also used `ERROR3` and `ERROR4` in the test files since there is another patch that has not
landed yet that uses `ERROR1` and `ERROR2`: https://reviews.llvm.org/D134001