Add some convenience accessors for the underlying Use of an operand.
These complement many of the existing accessors and make it
significantly easier to write code which needs to poke at the underlying
Use without hard coding the operand number at which it resides for
a particular instruction. No functionality changed of course.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@202102 91177308-0d34-0410-b5e6-96231b3b80d8
[SROA] Use a more direct way of determining whether we are processing
the destination operand or source operand of a memmove.

It so happens that it was impossible for SROA to try to rewrite
self-memmove where the operands are *identical*, because either such
a think is volatile (and we don't rewrite) or it is non-volatile, and we
don't even register it as a use of the alloca.

However, making the 'IsDest' test *rely* on this subtle fact is... Very
confusing for the reader. We should use the direct and readily available
test of the Use* which gives us concrete information about which operand
is being rewritten.

No functionality changed, I hope! ;]

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@202103 91177308-0d34-0410-b5e6-96231b3b80d8