[InstCombine] fold bitcasts around an extractelement (3rd try)
This is a redo of r255137 (reverted at r255227) which was a redo of
r255124 (reverted at r255126) with a fixed check for a scalar source
type and an added test for the failure that caused the revert.
Original commit message:
bitcast (extractelement (bitcast <2 x float> %X to <2 x i32>), 1) to float ---> extractelement <2 x float> %X, i32 1
This is part of fixing PR25543:
The next step will be to generalize this fold:
trunc ( lshr ( bitcast X) ) -> extractelement (X)
Ie, I'm hoping to replace the existing transform of:
bitcast ( trunc ( lshr ( bitcast X)))
with 2 less specific transforms to catch the case in the bug report.
Differential Revision: http://reviews.llvm.org/D14879