GlobalISel: improve representation of G_SEQUENCE and G_EXTRACT

GlobalISel: improve representation of G_SEQUENCE and G_EXTRACT

First, make sure all types involved are represented, rather than being implicit
from the register width.

Second, canonicalize all types to scalar. These operations just act in bits and
don't care about vectors.

Also standardize spelling of Indices in the MachineIRBuilder (NFC here).

llvm-svn: 279294