[PowerPC] Re-enable combine for i64 BSWAP on targets without LDBRX

The combine was disabled in 4e22c726 as it caused failures in
the ppc64be-multistage (bootstrap) bot.
It turns out that the combine did not correctly update the MMO for
the high load which caused aliased stores to be reported as unaliased.
This patch fixes that problem and re-enables the combine.