reflect: fix typo in result-in-registers case

reflect: fix typo in result-in-registers case

t is the type of the function that was called
tv is the type of the result

This fixes the failures for
GOEXPERIMENT=regabi,regabiargs go test go test text/template
GOEXPERIMENT=regabi,regabiargs go test go test html/template

Updates #40724.

Change-Id: Ic9b02d72d18ff48c9de1209987cc39da619c2241
Reviewed-on: https://go-review.googlesource.com/c/go/+/308189
Trust: David Chase <drchase@google.com>
Run-TryBot: David Chase <drchase@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Austin Clements <austin@google.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>