Previously, CodeStubAssembler macros performing FixedArray element accesses had
to compute offsets to elements explicitly with a fair amount of duplicated
code. Furthermore, any peephole optimizations that could produce better code--
like recognizing constant indices or combining array index computation with Smi
untagging--were also duplicated.

This change factors the code to compute FixedArray index offsets into a common
routine in the CodeStubAssembler that applies standard peephole optimizations to
all accesses. In order to do this, it also introduces limited introspection into
the up-until-now opaque Node* type exported from code-assembler.h, allowing
Nodes to be queried whether they are constant and extracting their constant
value in that case.

Review-Url: https://codereview.chromium.org/1989363004
Cr-Commit-Position: refs/heads/master@{#36370}