Revert "[SelectionDAG][Mips][PowerPC][RISCV][WebAssembly] Teach...

Revert "[SelectionDAG][Mips][PowerPC][RISCV][WebAssembly] Teach computeKnownBits/ComputeNumSignBits about atomics"

This seems to have broken sanitizers, giving lots of

  Assertion `NumBits <= MAX_INT_BITS && "bitwidth too large"' failed.

failures across multiple targets (currently X86 and PowerPC). Reverting
until I have a chance to reproduce and debug.

This reverts commit 6e876f9d.