A cast<> should have been dyn_cast<>.
Add basic support for creating floating-point constants.
Rename IceCfg::getConstant to getConstantInt for clarity and consistency.
Add high-level ICE support for the Fcmp instruction.
Add separate FileCheck commands for "ICE translation error" messages.

It turns out the CHECK-NOT lines at the end of each file only check
for ICE translation error messages that come after the last CHECK
line, whereas we really want to check the entire output regardless of
the normal CHECK lines.  We fix this by running a separate FileCheck
that just checks for the error message.
Lower floating-point arithmetic instructions.

Since the floating-point xmm registers aren't yet represented,
register allocation and code emission will improperly use the integer
registers.  This will be fixed later.
Lower float-point cast instructions.
Use a proper set of x86 condition mnemonics.

Previously we were reusing the LLVM Icmp conditions for all the x86
conditional branches, but there aren't enough of the LLVM ones to deal
with the Fcmp conditions, particular the ones that deal with NaNs.
Lower the Fcmp instruction.
Add floating-point tests.

This is just the skeleton that tests against translation failures.
Actual code pattern tests need to be added.