Remove unnecessary extra version of isValidAssumeForContext. NFC.

There were 2 versions of this method.  A public one which takes a
const Instruction* and a private implementation which takes a mutable
Value* and casts to an Instruction*.

There was no need for the 2 versions as all callers pass a const Instruction*
and there was no need for a mutable pointer as we only do analysis here.

llvm-svn: 278434