[ARM] in LowerConstantFP, make sure we cover armv6-m execute-only

Currently in LowerConstantFP, when we compile for execute-only (XO) we don't
check what architecture we're compiling for (v6m=< or >v6m). We shouldn't get
here for v6m, so put in an assert.

Reviewed By: simonwallis2, dmgreen

Differential Revision: https://reviews.llvm.org/D154506