remove micro-optimizations in HTML lexer

remove micro-optimizations in HTML lexer

This code was attempting to avoid an expensive call to strcasecmp when the first
byte already indicates a mismatch. Modern compilers can perform this
optimization themselves, including inlining strcasecmp and unrolling and
vectorizing its loop. Manually comparing the first byte actually impedes the
compiler because it's harder for it to see your intent. Related to #1913.