[KnownBits] Remove asserts from KnownBits::makeNegative/makeNonNegative

Many of the callers don't guarantee there is no conflict before calling these and instead check for conflicts later.

The makeNegative/makeNonNegative methods replaced Known.One.setSignBit() and Known.Zero.setSignBit() calls that didn't have asserts originally. So removing the asserts is no worse than the original code.

Fixes PR35769

llvm-svn: 321539