cmd/compile: avoid reading entire PGO profile just to check the header
isPreProfileFile reads the entire file into memory just to check the
first few bytes, and then throws it all away. We can avoid this by just
peeking at the beginning.
For #58102.
Change-Id: Id2c2844e5e44a2f3a9c7cdb9a027d94d26bdf71d
Reviewed-on: https://go-review.googlesource.com/c/go/+/560035 Reviewed-by: Cherry Mui <cherryyz@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Michael Pratt <mpratt@google.com>