GlobalISel: implement low-level type with just size & vector lanes.

GlobalISel: implement low-level type with just size & vector lanes.

This should be all the low-level instruction selection needs to determine how
to implement an operation, with the remaining context taken from the opcode
(e.g. G_ADD vs G_FADD) or other flags not based on type (e.g. fast-math).

llvm-svn: 276158