Simplify const-string.indexOf().

Simplify String.indexOf() called on empty or single-char
constant string. We see these patterns in inlined
    java.net.URI$Parser.scan(int, int, String, String)
called with constant strings from other URI$Parser methods.
The empty string simplification allows constant folding
and DCE to remove some of the code. The single-character
string simplification avoids entrypoint call overhead.

Test: New tests in 458-checker-instruct-simplification
Test: m test-art-host-gtest
Test: testrunner.py --host --optimizing
Change-Id: I4d3c997a8d6220202d481bbf8cbf280832c27cd7
2 files changed