Introduce GT_JCC.

Introduce GT_JCC.

This node represents a jump that is conditional upon the value stored in
the target's condition code register. It is only valid in the backend.
No formal modeling of the CCR is performed, so its use must be
constrained such that instructions that def the CCR are not inserted
between the JCC node and the node that it expected to def the CCR.

This is currently only used when lowering compares of long-typed
values for x86.

Commit migrated from https://github.com/dotnet/coreclr/commit/4351a27c18a88c50c0e701975d9a58f5ade57e78