Most of the subtlety here comes from needing to deal with either the old
parts or the new being possibly a constant object.

Uses WalkLiteralAccessChain combined with the size of the "new part"
to determine the range within the object which should be replaced.

Bug: b/126475423
Change-Id: I81f58d0de5dcacc74ffbd155708996c23dbe388e
Reviewed-on: https://swiftshader-review.googlesource.com/c/25214Tested-by: Chris Forbes <chrisforbes@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Ben Clayton <bclayton@google.com>
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>