[GlobalISel] Update the documentation for G_SEQUENCE
[GlobalISel] Update the documentation for G_SEQUENCE

This instruction has been removed in r306120.

NFC.

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@314167 91177308-0d34-0410-b5e6-96231b3b80d8
(cherry picked from commit 17f687fd5a22905a8e56e1e1d12af8ff3c699576)

apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm