[ValueTracking] Don't delete assumes of side-effectful instructions

ValueTracking has to strike a balance when attempting to propagate information
backwards from assumes, because if the information is trivially propagated
backwards, it can appear to LLVM that the assumption is known to be true, and
therefore can be removed.

This is sound (because an assumption has no semantic effect except for causing
UB), but prevents the assume from allowing further optimizations.

The isEphemeralValueOf check exists to try and prevent this issue by not
removing the source of an assumption. This tries to make it a little bit more
general to handle the case of side-effectful instructions, such as in

  %0 = call i1 @get_val()
  %1 = xor i1 %0, true
  call void @llvm.assume(i1 %1)

Patch by Ariel Ben-Yehuda, thanks!

Differential Revision: https://reviews.llvm.org/D36590

llvm-svn: 310859