[llvm-commits] [patch] Improve register coalescing

[Rafael Ávila de Espíndola]

This patch improves register coalescing so that it doesn't give up on 
coalescing A and B if a BB contains:

A = X
B = X

Instead, we pretend that it was

A = X
B = A

This helps a lot in some degenerate cases, but help even in more common 
ones. A self hosted clang goes from 33627704 to 33626776 bytes, but 
jsinterp.o in firefox goes from 2069728 to 1598504 bytes.

This also speeds up the hard case. llc on jsinterp.bc goes from 23.17s 
to 13.34s.

Cheers,
Rafael
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