cmd/gc: add temporary-merging optimization pass

The compilers assume they can generate temporary variables
as needed to preserve the right semantics or simplify code
generation and the back end will still generate good code.
This turns out not to be true. The back ends will only
track the first 128 variables per function and give up
on the remainder. That needs to be fixed too, in a later CL.

This CL merges temporary variables with equal types and
non-overlapping lifetimes using the greedy algorithm in
Poletto and Sarkar, "Linear Scan Register Allocation",
ACM TOPLAS 1999.

The result can be striking in the right functions.

Top 20 frame size changes in a 6g godoc binary by bytes saved:

5464 1984 (-3480, -63.7%) go/build.(*Context).Import
4456 1824 (-2632, -59.1%) go/printer.(*printer).expr1
2560   80 (-2480, -96.9%) time.nextStdChunk
3496 1608 (-1888, -54.0%) go/printer.(*printer).stmt
1896  272 (-1624, -85.7%) net/http.init
2688 1400 (-1288, -47.9%) fmt.(*pp).printReflectValue
2800 1512 (-1288, -46.0%) main.main
3296 2016 (-1280, -38.8%) crypto/tls.(*Conn).clientHandshake
1664  488 (-1176, -70.7%) time.loadZoneZip
1760  608 (-1152, -65.5%) time.parse
4104 3072 (-1032, -25.1%) runtime/pprof.writeHeap
1680  712 ( -968, -57.6%) go/ast.Walk
2488 1560 ( -928, -37.3%) crypto/x509.parseCertificate
1128  392 ( -736, -65.2%) math/big.nat.divLarge
1528  864 ( -664, -43.5%) go/printer.(*printer).fieldList
1360  712 ( -648, -47.6%) regexp/syntax.(*parser).factor
2104 1528 ( -576, -27.4%) encoding/asn1.parseField
1064  504 ( -560, -52.6%) encoding/xml.(*Decoder).text
 584   48 ( -536, -91.8%) html.init
1400  864 ( -536, -38.3%) go/doc.playExample

In the same godoc build, cuts the number of functions with
too many vars from 83 to 32.

R=ken2
CC=golang-dev
https://codereview.appspot.com/12829043