GVN-hoist: improve code generation for recursive GEPs

When loading or storing in a field of a struct like "a.b.c", GVN is able to
detect the equivalent expressions, and GVN-hoist would fail in the code
generation.  This is because the GEPs are not hoisted as scalar operations to
avoid moving the GEPs too far from their ld/st instruction when the ld/st is not
movable.  So we end up having to generate code for the GEP of a ld/st when we
move the ld/st.  In the case of a GEP referring to another GEP as in "a.b.c" we
need to code generate all the GEPs necessary to make all the operands available
at the new location for the ld/st.  With this patch we recursively walk through
the GEP operands checking whether all operands are available, and in the case of
a GEP operand, it recursively makes all its operands available. Code generation
happens from the inner GEPs out until reaching the GEP that appears as an
operand of the ld/st.

Differential Revision: https://reviews.llvm.org/D22599

llvm-svn: 276841