cmd/compile: untyped constant to complex conversion crashes compiler

[griesemer]

go build crashes for:

package a
var x complex64 = 774516156475830989403545856475830078125

panic: interface conversion: interface {} is *gc.Mpcplx, not *gc.Mpflt

goroutine 1 [running]:
cmd/compile/internal/gc.overflow(0x379d00, 0x820b7ac80, 0x820a848c0)
    /Users/gri/go/src/cmd/compile/internal/gc/const.go:449 +0x55f
cmd/compile/internal/gc.defaultlit(0x820a9add8, 0x820a848c0)
    /Users/gri/go/src/cmd/compile/internal/gc/const.go:1295 +0x3b7
cmd/compile/internal/gc.assignconvfn(0x820b79200, 0x820a848c0, 0x820a9adf8, 0x0)
    /Users/gri/go/src/cmd/compile/internal/gc/subr.go:1335 +0xfd
cmd/compile/internal/gc.assignconv(0x820b785a0, 0x820a848c0, 0x4192c0, 0xa, 0x820a9ae58)
    /Users/gri/go/src/cmd/compile/internal/gc/subr.go:1320 +0x62
cmd/compile/internal/gc.typecheckas(0x820b78630)
    /Users/gri/go/src/cmd/compile/internal/gc/typecheck.go:3324 +0x11f
cmd/compile/internal/gc.typecheck1(0x820a9b9f8, 0x2)
    /Users/gri/go/src/cmd/compile/internal/gc/typecheck.go:2055 +0x490b
cmd/compile/internal/gc.typecheck(0x820b4d4c0, 0x2, 0x0)
    /Users/gri/go/src/cmd/compile/internal/gc/typecheck.go:202 +0x939
cmd/compile/internal/gc.Main()
    /Users/gri/go/src/cmd/compile/internal/gc/lex.go:385 +0x2126
cmd/compile/internal/amd64.Main()
    /Users/gri/go/src/cmd/compile/internal/amd64/galign.go:127 +0x58d
main.main()
    /Users/gri/go/src/cmd/compile/main.go:26 +0x189

[gopherbot]

CL https://golang.org/cl/14596 mentions this issue.