[AArch64][GlobalISel] Fix a crash during s128 G_ICMP legalization due to r366317.

r366317 added a legalization for s128 G_ICMP narrow scalar which tried to hard
code the result type of the new legalized G_SELECT. Change this to instead use
type of the original G_ICMP result and allow the target to legalize it if necessary
later.

llvm-svn: 366943