cmd/gc: allocate non-escaping maps on stack

Extend escape analysis to make(map[k]v).
If it does not escape, allocate temp buffer for hmap and one bucket on stack.

There are 75 cases of non-escaping maps in std lib.

benchmark                                    old allocs     new allocs     delta
BenchmarkConcurrentStmtQuery                 16161          15161          -6.19%
BenchmarkConcurrentTxQuery                   17658          16658          -5.66%
BenchmarkConcurrentTxStmtQuery               16157          15156          -6.20%
BenchmarkConcurrentRandom                    13637          13114          -3.84%
BenchmarkManyConcurrentQueries               22             20             -9.09%
BenchmarkDecodeComplex128Slice               250            188            -24.80%
BenchmarkDecodeFloat64Slice                  250            188            -24.80%
BenchmarkDecodeInt32Slice                    250            188            -24.80%
BenchmarkDecodeStringSlice                   2250           2188           -2.76%
BenchmarkNewEmptyMap                         1              0              -100.00%
BenchmarkNewSmallMap                         2              0              -100.00%

benchmark                old ns/op     new ns/op     delta
BenchmarkNewEmptyMap     124           55.7          -55.08%
BenchmarkNewSmallMap     317           148           -53.31%

benchmark                old allocs     new allocs     delta
BenchmarkNewEmptyMap     1              0              -100.00%
BenchmarkNewSmallMap     2              0              -100.00%

benchmark                old bytes     new bytes     delta
BenchmarkNewEmptyMap     48            0             -100.00%
BenchmarkNewSmallMap     192           0             -100.00%

Fixes #5449

Change-Id: I24fa66f949d2f138885d9e66a0d160240dc9e8fa
Reviewed-on: https://go-review.googlesource.com/3508
Reviewed-by: Keith Randall <khr@golang.org>
Run-TryBot: Dmitry Vyukov <dvyukov@google.com>

test/escape2n.go

@@ -1751,3 +1751,20 @@ func slicerunetostring2() {
 	r := []rune{1, 2, 3} // ERROR "\[\]rune literal does not escape"
 	sink = string(r)     // ERROR "string\(r\) escapes to heap"
 }
+
+func makemap0() {
+	m := make(map[int]int) // ERROR "make\(map\[int\]int\, 0\) does not escape"
+	m[0] = 0
+	m[1]++
+	delete(m, 1)
+	sink = m[0]
+}
+
+func makemap1() map[int]int {
+	return make(map[int]int) // ERROR "make\(map\[int\]int\, 0\) escapes to heap"
+}
+
+func makemap2() {
+	m := make(map[int]int) // ERROR "make\(map\[int\]int\, 0\) escapes to heap"
+	sink = m
+}