[LSR] Handle case 1*reg => reg. PR50918

This patch addresses assertion failure in case when the only found formula for LSR
is `1*reg => reg` which was supposed to be an impossible situation, however there
is a test that shows it is possible.

In this case, we can use scale register with scale of 1 as the missing base register.

Reviewed By: huihuiz, reames
Differential Revision: https://reviews.llvm.org/D105009