cmd/compile/internal/syntax: use strings.LastIndexByte in trailingDigits

cmd/compile/internal/syntax: use strings.LastIndexByte in trailingDigits

Previously, strings.LastIndexByte couldn't be used because it was only
added in Go 1.5 but Go 1.4 was required for bootstrapping. In Go 1.18,
the bootstrap toolchain was bumped to Go 1.17 (see #44505), thus
strings.LastIndexByte can be used now.

Change-Id: I01a70a59dbfc853cf03d49747a2dd62d21ba74e9
Reviewed-on: https://go-review.googlesource.com/c/go/+/522197
Reviewed-by: Ian Lance Taylor <iant@google.com>
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Auto-Submit: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>