The final condition (db.maxIdleConnsLocked() > db.freeConn.Len()) can
only be true iff db.maxIdleConnsLocked() is greater than 0, so previously
checking if it's greater than 0 is a waste, specially when that involves
a method call which (ATM) can't be inlined and includes a switch.

Dissasembly follows (test for err == nil has been omitted for clarity):

Before:
43c357: cmp    $0x0,%bl
43c35a: jne    43c3ce <database/sql.(*DB).putConnDBLocked+0x1ce>
43c35c: mov    %rax,(%rsp)
43c360: callq  43aec0 <database/sql.(*DB).maxIdleConnsLocked>
43c365: mov    0x8(%rsp),%rbx
43c36a: cmp    $0x0,%rbx
43c36e: jle    43c3ce <database/sql.(*DB).putConnDBLocked+0x1ce>
43c370: mov    0x30(%rsp),%rbx
43c375: mov    %rbx,(%rsp)
43c379: callq  43aec0 <database/sql.(*DB).maxIdleConnsLocked>
43c37e: mov    0x30(%rsp),%rdx
43c383: mov    0x8(%rsp),%rcx
43c388: mov    0x28(%rdx),%rbp
43c38c: mov    0x28(%rbp),%rbx
43c390: cmp    %rcx,%rbx
43c393: jge    43c3ce <database/sql.(*DB).putConnDBLocked+0x1ce>
43c395: mov    0x28(%rdx),%rbp
43c399: mov    %rbp,(%rsp)
43c39d: mov    0x38(%rsp),%rcx
43c3a2: mov    $0x556c60,%eax
43c3a7: mov    %rax,0x8(%rsp)
43c3ac: mov    %rcx,0x10(%rsp)
43c3b1: callq  4db5b0 <container/list.(*List).PushFront>

After:
43c357: cmp    $0x0,%bl
43c35a: jne    43c3b5 <database/sql.(*DB).putConnDBLocked+0x1b5>
43c35c: mov    %rax,(%rsp)
43c360: callq  43aec0 <database/sql.(*DB).maxIdleConnsLocked>
43c365: mov    0x30(%rsp),%rdx
43c36a: mov    0x8(%rsp),%rcx
43c36f: mov    0x28(%rdx),%rbp
43c373: mov    0x28(%rbp),%rbx
43c377: cmp    %rcx,%rbx
43c37a: jge    43c3b5 <database/sql.(*DB).putConnDBLocked+0x1b5>
43c37c: mov    0x28(%rdx),%rbp
43c380: mov    %rbp,(%rsp)
43c384: mov    0x38(%rsp),%rcx
43c389: mov    $0x556c60,%eax
43c38e: mov    %rax,0x8(%rsp)
43c393: mov    %rcx,0x10(%rsp)
43c398: callq  4db590 <container/list.(*List).PushFront>

R=golang-dev, bradfitz, iant
CC=golang-dev
https://golang.org/cl/14656044