UNPACK the size field of SizedSeq
UNPACK the size field of SizedSeq

(cherry picked from commit 1fe5c8916a2bdf90ed2d9c70f900642b56650592)

GaloisInc/halvm-ghc,GaloisInc/halvm-ghc,GaloisInc/halvm-ghc,GaloisInc/halvm-ghc,GaloisInc/halvm-ghc,GaloisInc/halvm-ghc,GaloisInc/halvm-ghc