When emiting a CopyFromReg and the source is already a vreg, do not bother creating a new vreg and inserting a copy: just use the input vreg directly.
When emiting a CopyFromReg and the source is already a vreg, do not bother
creating a new vreg and inserting a copy: just use the input vreg directly.

This speeds up the compile (e.g. about 5% on mesa with a debug build of llc)
by not adding a bunch of copies and vregs to be coallesced away.  On mesa,
for example, this reduces the number of intervals from 168601 to 129040
going into the coallescer.


git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@23671 91177308-0d34-0410-b5e6-96231b3b80d8

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