swift234

Re-apply "IRGen: Fix stupid oversight in emission of fixed multi-payload enum vw_getEnumTag()"

Fixes two issues with the original patch:

- If numCaseBits was >= 32, the high bits of the tag were shifted
  by an incorrect amount. In this case, the high bits were always
  zero anyway, but the invalid shift triggered an assertion in
  ARM64 codegen. Fixed to not generate the offending instructions
  at all, since just loading the payload value is enough.

- When the payload is very small, the number of no-payload cases
  could be greater than 2**payloadBits, requiring multiple payload
  tags to represent them all. There was an arithmetic error in
  this case.

This re-applies r31328.

Swift SVN r31365